Cold Start Problem + misfire around 2000rpm......

I have a 1981 XJ750 Seca


It's really really hard to start when it's cold after sitting overnight..... the warmer it is the easier it is to start. If it sat for 15 minutes it will start instantly.


It always sounds like it wants to catch on the first crank but it never does.
I usually have to crank her a good 10 seconds before it would dare to catch. Seems to me that the cranking is warming her up.

it usually tries to die after a second or two.... even with the choke on.


So I go at her again and after another 5 seconds or so it'd usually catch on again and this time it usually stays running as long as i rev her up gently and have the chock on good so it sits well above idle untill it's warmed up.

She does seem to have a bit of a lower idle.... hard to tell what rpm she's exactly on when idling warm.

but whenever I open the throttle wide at 2000rpm, she bogs like no tomorrow. misfire and just stuck there at 2000rpm trying hard to climb. After about 3-5 seconds of that it'd get up and take off.

I checked the plugs, they're pretty fresh. I bought this bike less than a month ago and the previous owner must have dropped some new plugs in.

The only problem is that the wires have resister in them and they don't sell BP7ES in Canada anymore. So the plugs in her are BPR7ES which are resisted plugs. I called Yamaha and they said they drop BPR7ES on all the bikes all day long including the ones that should have BP7ES and they run just fine.

I also checked the gap and the plugs are sitting at around 0.023-0.025. I know that the specs state 0.028-0.032.
I left them alone because I thought maybe the resistance was causing them to require a smaller gap....


Any thoughts? Try some new BPR7ES and hope it makes a difference? gap them to 0.030? order BP7ES from the states? Try another brand of plugs??


I used to drive a vw 1.8t boosting 21 psi and pushing 600hp. One thing I learned was that high pressure + high heat = dead plugs. I used iridium plugs and replaced them every 5000km because they'd start misfiring.
I also found out that the best way to know if it's the plugs when it's misfiring without spending $80 on new iridium plugs was to gap them a little closer. If the misfire goes away mostly but comes back after a few hundred km them it's the plugs. Closer gap means easier to spark so that made sense...


I don't feel comfortable closing the gap anymore than the way they already are at 0.023 But if I open them up to 0.030 then it wouldn't tell me whether my plugs are dieing or that I simply need to have a smaller gap if i run resisted plugs on resisted wires.....

 

Get the BP7ES and gap them correctly. I have seen other members on here acquire bikes with the resistor plugs, and it never ran well till they put the proper plugs in. You could also try removing the resistor caps from the wires, and try running the resistor plugs that way, but the gap still needs to be set right. Personally, I would just hit Chacal up for a set or the correct plugs.

 

-check the valve clearances.

-clean the carbs, and properly WET SET THE FLOAT LEVELS (this is likely the problem based on symptom) and bench sync them,

-vacuum sync the carbs, with YICS blocked. But you can't sync until the valves are in spec; and it won't run for squat if the float levels are off.

You need to correctly gap the plugs, and if you can find some non-resistor BP7ES go for it; but you do need to get them gapped correctly.

And as I said in my response to your original thread, a 550 Seca/650 Seca front fender will fit correctly and not stick up in the air like that. The reason the previous owner swapped front ends was probably to get rid of the rectangular headlight/running light, remote-op master cylinder and cast handlebars. But then he stuck the 750 fender back on.

 

ok it got A LOT WORSE!!!
Not sure if it's the rain today or what but its misfiring all the way from idle to 3000rpm!!!

feels like i'm running on 3 cylinders almost....


where is "chacal"? I called every shop in town and nobody has any non resisted plugs so I'm planning on making a trip down to Seattle to get some plugs. I'd call ahead to make sure they do have them..... it's a good 3 hour ride there..... bummmmer

I'll have the gap corrected.

I also just remembered that when my plugs went foul in my cars, they always misfire at the highest rpms and work their way down.... not up...


And I know nothing about carbs.
any easy way out as far as carbs go? seafoam? or easy way to diagnose it for sure?

The valve clearance was set a couple thousand kms ago and they were perfect at that time. I have a valve cover gasket being shipped, arriving this week hopefully. Will check valve clearance again at that point, but the valves sounds good to me so far.....


Do i need any special tools to check the valve clearance or special instructions? Or just like a car? roll the cam so the lump is pointed away from the valve and stick a feeler gauge in there till i get one thats too thick to go in.....

 

FWIW, I am running Autolite 63 spark plugs in my 82 XJ750RJ and it purrs like a kitten. Also, thanks to the wonders of fuel injection, I too am not so experienced with carburetors, but I did just rebuild my Hitachi's. I even performed the dreaded wet set. If you can use tools and follow directions, it's just not that difficult. Preparation and patience are the keys.

I would however, seek some guidance on the effect of your aftermarket air filters and exhaust on the jetting/tuning requirements of your carbs. With the issues you're experiencing, it makes me wonder if the previous owner addressed those concerns.

I saw pics of your bike in a previous post. Looks nice; like someone took alot of time on the cosmetics. With some quality tuning and wrenching time, you should have a real jewel.

 

Seafoam won't fix carbs that need cleaning or out of spec float levels. Depending on your problem, it could make the bike run worse. I wouldn't.

Valve clearances: http://xjbikes.com/Forums/viewtopic/t=14827.html

Part Deux: http://xjbikes.com/Forums/viewtopic/t=29209.html

Cleaning carbs: http://xjbikes.com/Forums/viewtopic/t=14692.html

and: http://xjbikes.com/Forums/viewtopic/t=2908.html

Setting float levels: http://www.xj4ever.com/setting%20fuel%20levels.pdf

All of the above were intended as supplements to a SERVICE MANUAL. You're going to need a book. The Haynes is fine, the factory book better and the best thing is a combo of the two.

Forum member chacal is the proprietor of XJ4Ever, and is our resident parts source. His "catalog" is in the "XJ4Ever-Supporting Vendor" forum or you can click the logo/link in the site banner at upper right. Send him a PM or email 'info AT XJ4Ever DOT com' with your parts needs.

If you're going to run down to the 'States to buy plugs, AutoZone generally carries them for under $2. Buy a box of 8.

If moisture is affecting the issue; pull the tank, and clean off the coils. Examine for cracks, or rust/corrosion at their grounding posts/mounts. Check all wiring/connections.

Remove the spark plug caps from the wires-- they unscrew off. There is a threaded spike in the cap that screws into the core of the wire. Lop off a CM or so (1/4"~1/2") and screw the cap back onto/into the wire. Don't overtighten and strip it now that it has a fresh "bite."

Look INSIDE the plug caps where the top of the spark plug goes. There is a resistor core that screws into the cap. On either side of the hole for the plug contact are screwdriver slots. Gently ensure the inner core has not come loose.

You can do this right now, while you're re-gapping the plugs.

This is the bike with "painted as a unit" motor, isn't it? I wouldn't trust anything anyone has told you in regard to valve clearances, etc. Same for the alternator brushes. None of the covers have been "cracked" open since the mill was painted; and you only have somebody's WORD about what shape it was in before the paint job.

Definitely get the valve clearances checked, that one cylinder is right at the limit in comparison to the others, compression-wise. It could just be tight valves.

Whoever had this bike prior was more concerned with cosmetics than mechanics. You might wanna check the rear brake too (don't take somebody's word for it) http://xjbikes.com/Forums/viewtopic/t=15874.html

 

That too. But everything else needs to be 100% before trying to tune for the pods and pipe. And I doubt very much that it is.

 

Mine was doing something similar to that. I would have to rev past 3k to pull away from a stop, other wise it would just bog down.

I realized I was running super lean. My floats were set too low, and my main air jet was oversized.

 

The previous owner didn't ride her much at all. he had the motor rebuilt a couple years back cuz of a leaky valve and thats when it got repainted.

He had the exhaust and intake on her since the rebuild and she's been rejetted to run with them. She was running perfectly fine when i bought her. just a little tricky to start when cold. nothing else

 

You CAN convert those resistor caps to non resist caps (I had to do the same thing this past year from lack of BP7ES in Canada)

Unscrew that inner core Fitz was talking about, a spring and a "resistor" will come out, replace that resistor with and same sized piece of solid metal (think 3/4" of an allen wrench cut down) put it in with the spring and enjoy non resistive caps.

I should add here, when i did my conversion, for some reason, when i measured resistance on the cap after the mod, it would read open. I then put all 4 modded caps on the bike and she fired right up.

So even if you check the caps and they read open circuit try them anyways. This could mean that the spark is jumping a very close gap between the metal and spring, I would think that after a few hits it would weld itself and be fine after.

I never did re measure my caps to see if the resistance changed to 0 Ohm because i never had any issues after doing the mod (If it ain't broke...)

 

You already have your answer:

 

it's not the carb or the valves for sure.

ok i'm not 100% sure they're working perfectly... I'm doing my valve cover gasket next week so i'll check on the valves while i'm at it

but it don't make sense that the valves or the carbs would go from running like a champ to stalling and misfiring within 1000km....


THE MAIN REASON why i'm saying its neither the carbs nor the valves is cuz shes been running PERFECTLY the past few days!

I haven't had to take her out in the rain again yet so the past few days of riding have been dry days and not a single misfire.
Crank up the choke and press start and she'd start right up. But she'd stall after 10 seconds. Seems to catch on after the third try.


i have a feeling the hard start is the plugs. and i'm probably dealing with an electrical issue here........

What do you guys think?

 

Change your plug caps to non resistor and that will help since you can't find non resistor plugs.

MN

 

Picked up proper non resisted plugs, gapped properly. Still hard to start when cold.

Havn't had the misfire problem at all lately. Rode half an hour in light rain a couple times and still no problem.... hope it stays that way.

Now this is my first motorcycle so forgive me if i sound stupid, but normally when you go at it first thing in the morning, you open the choke/enricher fully and you start her up. She runs at idle and she's supposed to stay so untill warms up a bit then rev up higher correct??
Mine idles for 5 seconds? then it dies.

Lately it got a lot colder here so the second, third, forth, fifth tries are all pretty sad..... I find that I got more hope if i let it sit for 5 seconds or more between each attempt and they'd idle for maybe 3 seconds. Sometimes it takes me a good dozen times before it'd stay idleing and warm up.....


I got my valve cover seal so just waiting to have some time off to pop her open. but really not too keen on rebuilding a carb. I got no clue how to tune her and not exactly something I'd wanna do unless i absolutely have to....

 

Sounds like the proper plugs helped out your misfire issue. But you still have dirty carbs that need cleaned. Your enrichment circuit is plugged, or it would work properly. The reason it is dying shortly after idle is likely that there isn't enough fuel getting thru the enrichment circuit. The carbs are really easy to clean/rebuild, it is just time consuming and you have to be organized and meticulous. Take your time and be thorough and you will be greatly rewarded with a proper and smooth running XJ.

 

The enrichment does work, least once it's slightly warmed up...



And YES the plugs did help. First couple of rides I didn't notice but i'm definetely getting a smoother idle and runs better overall with less misfire.
Also starts up cold a lot easier now. Couple tries if not just the first try. but it did get warmer again here in Vancouver....

Do I have to rejet or colortune or anything when i clean the carbs?

Or is it a simple surgery where I pull things off to take the carbs off then sit down on my work bench with good lighting and just take things off one at a time to clean with carb cleaner and tooth brush/paintbrush/toothpick??? Then reassemble in reverse manner and tada! kinda thing?

Is the idea similar to a throttle body, where carbon builds up and old cars which has a secondary idle valve gets clogged and becomes a mess? I'd pull my carbs off and it's gunked with carbon and i take my cleaner and brush it all off except a LOT more parts which are A LOT smaller???

 

It's funny that you mention it ran fine right after you got it, but then it went south. I had a similar experience although mine wasn't totally fine when I got it, but it did develop issues shortly thereafter (~400mi). It developed a bad misfire at lower rpms, starting idling low, and plugs were lilly white (lean) and pilot adjustment didn't help. Like my bike yours is testing your commitment.

Fixed it by adjusting valves (was easy), cleaned carbs, restored stock air-box/filter, new plugs, fresh oil, new SS brake lines, new tires, new bearings, in-line filter, cleaned all electrical connections, new brakes, made it look nice! There's more but that's the most important.

The carbs aren't as bad as you might think or have been led to believe. The hardest part for me was getting them off and back on, the cleaning was time consuming but not bad.

Yes surgery in well lit spot with something to keep the parts organized (I used a muffin tin and some plastic bags). Yes start with 1 carb and do it completely then the rest. Yes take notes and digi pics helped me too. Carb cleaner is good, soaking any varnished metal parts is good, I used small pipe cleaners, some tiny plastic picks, 1000g paper, and q-tips to detail-finished with compressed air. If you plan on breaking the rack to do the throttle shaft seals it's a little more involved.

I didn't find any carbon, but I did come across gum & varnish and some strange black goo in my pilot circuit (main issue). Polished bores, 'clunk' tested, dry set floats by tilting bodies until just contacting and made measurements the same on all 4, bench synched, wet set, and adjusted pilots by using tiny increments, rev's, and then plug chops until good (took two).

I still need to color-tune and vac synch but I can't tell that it's needed. I'm getting 48-56mpg, no misfire, starts from cold straight away, barely needs enrichment (choke), plugs all tan and same, seems very smooth with almost no vibration. I took it to a race track nearby to wind it out afterwards and it topped out at 117mph (used GPS). It's only rated to run 113mph and it has over 30k!

When it is cold or damp it sometimes takes about 3 or 4 seconds to start instead of 1. I always switch to prime for a bit before firing, charge the battery about once a month, and keep a little upper cylinder lube (seafoam, MM, ethanol treatment) in the tank.

Since they are Japanese carbs, J spec screwdrivers are best, be careful with your brass parts (they be soft), and don't work anywhere that you can lose dropped parts. Don't work tired or frustrated, come here for help anytime.

....Whew...I think that about covers it. Now go for it and get that baby right!! It's about to be dank in V anyway and you'll have time to work on it. BTW can I come visit in the Summer, got relatives on the Island.

 

sorry its late here but quick question,

If i pull the carbs off and clean it, would that mess around with the tuning/adjustments or whatever there is?

as in are they supposed to remain untouched or did you get your's running well due to experience and luck?

Oh and how long did it take you?

I'm also thinking of taking the pods off and putting in the stock air box.... will that help?
according to the previous owner, they are jetted for the pods and i would say the bike ran spot on when i recieved it...... but then again i'm no bike expert. My guess is the air box would help stabilize things but means i'd have to rejet.....

 

rd337,

Welcome. Not trying to tell you anything you may already know but just a suggestion. I did some of the same thing a lot of us do on here and found out the "hard-way" as mentioned on here OVER AND OVER AGAIN. If you keep jumping on that bike and hoping it'll keep riding without doing the things necessary for it to ride properly you're going to regret it. In the long run you might end up getting "lucky" like I did and be able to coast home in first gear after I blew a hole in my third piston cap. Not trying to scare you, just giving you the hard facts. Bigfitz and some of the other guru members get a bit frustrated with us when they give us the answer and we don't listen. These bikes are 30yr old bikes and there aren't a lot of shops that will fool with them. The ones that will most of us don't have the money to pay what it would cost to have them work on them. I know you want your bike to run to form for a long time, just like the rest of us on here, you're going to have to do some wrenching. Once you start, like myself, and many of us will testify to, you will not regret it. Do those things that BF and MM have suggested and you will amaze yourself. There's no "tricks" here, nothing is "hidden". These members flat-out know these bikes. Everything you need to know is here. Use it. Not a rant, trying to build your confidence. We ALL need it from time to time.

Gary

 

Yes cleaning them means you will have to go through at the very least a very accurate bench synch, a wet set of the floats, and resetting your pilot screws. I just got lucky with mine, but it probably would be a smidge better after a colortune, and a vac synch.

The first carb took me about 4 hours to completely go through, and I took 2 breaks to keep my sanity and steady my hands. Mostly I was just paranoid about doing '4' carbs, and I didn't (at the time) have a back up rack so that made me even more noided. I had previously cleaned several single carbs (2 and 4 stroke) and did a friends dual carbs. The whole deal took me about 11 hours-but I was meticulous and intentionally took my time.

The pod thing has gone back and forth on here. I don't want to tell you it can't be done, because it has been done successfully. The main issue as I see it is the differences in the 'air-charge' between the 4 carbs. Having the OE box unifies this and makes tuning easier. Yes you would need to go back to stock jets if you go back to a stock box-unless your jets are close to stock and you run a free flow filter in the stock box-that's a gamble though.