engine with big bore kit won't start(well)

Big bore kit might have been a bad idea but i got the stuff for free.

anyways, so I have this bike engine that I rebuilt and now its a little under 820 cc but it is having a lot of trouble starting. now it will start eventually if you keep at it but I can decide if I have an fuel issue or electrical.

Symptoms starting bike:
The only way that I seem to be able to do to get this bike started is if I have the battery charged fully and a running car hooked up in parallel to the battery to give it a power boost. Then it will start (still not right away but it will start).

I should note that I am having some issues with my starter clutch slipping. As I turn it over I can get 4 rotations out of it before it starts to slip.(average estimate)

Now I originally thought I had coil pack issues so I swapped them out with some from a running bike and it didn't make a change on how hard it is to start the bike.

I have a feeling that I am getting a voltage drop somewhere in the system because while starting the bike I have checked the spark and its hard to see. I checked it by taking out a spark plug grounding it to the block and turning the bike over and made the observation that spark wasn't great. Also when I take my finger off the starter it will some times fire once. almost like the engine is still turning and because I am not sucking away power with the starter it can fire.

I have looked at the voltage across the battery when it starts and it drops way low.

my thought:
Starter need to be swapped out
spark plugs(need to be gaped right) what is the gap size for these bikes?
Pickup on crank is going bad.

any ideas?


thanks in advance you guys have always rocked.

 

1. Rebuild the starter.
2. Seeing a spark after releasing the start button is normal; part of how a TCI system functions.
3. Load-test the battery on the bike. What does the voltage drop to when you attempt to start the engine?
4. Did you re-jet for the larger displacement? If not, that is where your main trouble lies.

 

when trying to start voltage drop below 9.5 volts tci will not fire.
how old is the battery? could be bad
ohm out your ignition system
coils primary and secondary plug caps and pick up coils

specs and battery testing info found in this thread
The Ultimate Relay, Switch, Sensor, and Diodes Guide

I take it from the avatar photo your talking about a xj750


haynes manual says gap is 0.7mm or 0.028 inch

when it does start check header pipes for even heat it will tell you if you are firing on all cylinders

 

Another thing what compression ratio are running ? Too high can cause problems weak battery, dodgy starter , carbs wrong jets, out of synch all can add up to issues. Another question was bike running well before the change to bigger bore? This leads back potential problems with carbs...have they been to church?

 

HA, yea Its a 750 big detail I missed. Good thing you guys read minds

Before I rebuilt the bike it ran OK but had a tough time starting and its valve gaps were a mess which is why I decided to mess with it.


I rebuilt and cleaned the carbs. (bench synced them I know not great. need to actually sync them)

battery is new.
The voltage drops to really low like 7~9 volts maybe even lower its been a few months since I checked that out.
Its firing on all cylinders I am pretty sure.
Ill have to check out the ohms on everything and take a look at what I got.


I did re jet the carbs when I rebuilt them but didn't change them too much I can't remember any numbers for this off the top of my head. I'm not sure on how to go about determining what to jet the bike to or what size jet I need and where to find it.
SO what do I jet this thing to.
I do have low RPM torque issue from not setting up the fuel system right.

Compression ratio..... higher than before but I have not looked at my compression and I haven't recalculated the ratio. (I know Dumb)
Compression has gone way up. But I cant really get lower compression out of the bike without modifying and machining the heads. So do I just need to get a more powerful battery if that is the issue?

 

new batterys can be bad if you are getting 7 to 9 volts when cranking your bikes tci will not fire.

it could be you need a new battery or a larger cranking amps battery the 750 had a 14amp battery should be ok with that size
YB14L-A2

if you look at a 550 (about 525cc) it has the same battery as a 650 so a 750 to an 820 should work but I am comparing apples and oranges

 

To test this I will attempt to directly connect my Subaru battery and start the bike. bad idea? I can't see any troubles myself and that battery pushes electrons a lot better.

 

yes you can jump with a car battery just do not start the car. while you will likly get the bike started it will not tell you much about your bike battery. get one of those spicific battery testers a few bucks looks like an anti freeze tester when fully charged test the spicific gravity of the battery cells
you will find the info in this thread
The Ultimate Relay, Switch, Sensor, and Diodes Guide

it could be a bad starter that needs new brushes

i checked my manual for a xj900 it uses a battery rated at 12 amps but the battery number implies it is a 14 amp battery

 

Look in XJ PARTS CATALOG, SECTION C: FUEL SYSTEM and The Information Overload Hour for jetting info. Also do a googol search with site:xjbikes.com after your keywords for big bore. You'll be bumping both the mains and pilot jets up pretty close to what's need for an XJ900 (but not exactly since the 900 uses larger carbs).

If you find that you need to lower compression that can be done by using a second base gasket to raise the cylinders, but I doubt that you'll need to do that. I don't think that kit raises the compression ratio; what you're seeing is a new engine vs. a tired engine.

A new battery, and dialing in the jetting should sort the bike. The same starter is used on the XJ900, so it's plenty strong enough to turn that kit over.

 

Ok, tried to get the bike going and I couldn't get it started today. The starter clutch has become an even bigger issue. since it has sat for half a year without oil to the clutch I believe it has made it worse I'll post a video of me trying to get the thing started. My biggest issue now might be that starter clutch though it wasn't an issue before the big bore. Even with my car battery connected.

rechecked, voltage drop isn't the problem I thought it was. If the battery is fully charged it drops from 12.3 to 10.1 when the starter is engaged which should be enough.

I am less convinced it is electrical and I'm jumping onto fuel now I need to modify my fuel to air ratio by correctly re jetting the thing. Even if I have to start the bike by turning the crankshaft manually with a drill.

 

Are you running synthetic oil? This can cause problems with the starter clutch and main clutch .

 

12.3 volts is not fully charged. The resting voltage for a lead acid battery in good condition is 12.8 to 12.9 volts.
The starter clutch isn't harmed by sitting.
Replace the starter brushes if they've never been done by you. Get a new battery.

 

It did this before but not to this degree. could the compression be too much for it?
Cylinders got larger head didn't.

I just replaced the battery could it really be that bad?
I did dangerous and ran my car while jumped to my bike and it was at 14 volts in this video 12 while attempting to start. If that doesn't fix electrical issues I don't know what will. Ill try and trade in that battery.

I should note I got it to fire about 4 times but it just wasn't quite enough to start.

Im running oil that Auto zone sells that is for street bikes.

 

The starter clutch isn't slipping. It's being kicked back. I think a starter rebuild will be the place to start tending to being able to spin the engine fast enough. IF that' dosen't tak care of that part of the equation, then you'll need to look at dropping compression (my first thought is to add a second base gasket).

Running the car while trying to start the bike is a very bad idea. It is very possible for the car's alternator to push too many amps through and damage the voltage regualtor (among other things).

 

^^^^^+1 KMOE is right (AS usual lol) your starter needs new brushes.

 

One more thought and PLEASE do not take this wrong is your cam timing set correct ? if it is off it might run ,but poorly and can cause a starting problem . I've replaced timing belts in cars been off one tooth and can cause all kinds if fun. You might want to pull valve cover and double check , this could be an issue.

 

I like the sound of starter issue over starter clutch, that is for sure! I didn't want to have to pull the engine out. The carbs are the most irritating thing in the world to get on and off that bike. specifically the boots.

I don't think my cam timing is off. I checked it (while assembling the engine) and ran it. Then checked it again after it ran and it looked dead on. unfortunately I can do stupid. Ill check my timing. It would be dumb if that was it and I ignored the possibility.(as much as I don't want to).

one of the long bolts that hold the starter together is stripped so I am going to need to figure out how to get another so I can rebuild the thing. maybe ill just but a new starter.

you guys rock don't know if I mentioned it yet.

My course of action will now be. Figure out jets, fix starter, and make sure the carbs are in sync. Then hopefully i'm free from these troubles and I can finish the bike.

Yea and I knew it was a bad idea. But I can scratch things off the list now that I can be positive voltage isn't the issue.

 

For your starter broken bolt, try metric all thread cut to length and put a nut and lock washer on the end. If you were able to find a replacement bolt it would cost 10x the price.

 

OK, so I replaced this starter. I would just like to thank you for clearing up my starter problem. I was convinced it was the clutch (I don't know why). The new starter completely solved the issue with the slipping clutch. Not having to pull this engine to fix a starter clutch makes me very happy. Now I just need to deal with jetting the carburetors, welding on rear turning signals and tail light, and this project if finished.

I owe someone a beer. thanks

 

IPA please. Or a nice stout. . . .

 

Spaten Optimator.

 

Double up on spark plug washers to drop compression.

 

...and increase spark shrouding. Not a great solution in my opinion, but a cheap way to assess the situation.
Nevertheless, problem was solved by replacing the old, tired starter.

 

As for jetting, there are many discussions on the web on which direction to go after a big bore. Many convinced you will need to go down a size with the main jets. Yamaha throws cold water on that argument, look at the 750 and 650 jets. They both use the same carbs and the 750 does have larger jets. I'm not sure if they have same size air box, can't think of anything else which might play a part?
Based on my own experience I would see how it runs with the 750 jets and take it from there. Professional help is highly recommended as running it lean could quickly melt pistons. Dyno runs with exhaust gas analyzer will tell you instantly which way you need to go. You haven't stated but for your own sanity I hope you're starting with stock jetting and air box.

 

General rule of thumb: More bigger = sucks in more air = more fuel required.

The 650 and 750 airboxes are almost exactly the same size (back to that packaging issue being a limiting factor for airbox design).

 

The following question was sent in to Kevin Cameron at Cycle World. It is opposite of what Yamaha did with their XJ650/750.
I kept my jetting the same when I took my Maxim-X motor from 750 to 911 and it ran great but it was slightly rich prior to the big bore.
Bottom line - don't change anything until you put it on a Dyno with O2 measurement, or at least do a plug chop.

Question: Recently, I came across information which implied that when increasing displacement—such as installing a big-bore kit on a bike engine—if the carburetors stay the same throat diameter, the jetting might need to be leaner than that used with the previous smaller displacement. This seems counterintuitive. A larger displacement cylinder would require both more air and more fuel, so why would leaner jetting be needed?


Robb Millet

Milford, MA

Answer: I learned about this effect from the different cylinder sizes on three different Kawasaki two-stroke racers I worked with, all of which used the same-size 35mm carburetor. The 167cc cylinder of the H1-R 500 triple needed a 310-330 main jet, yet on the 250cc cylinder of the H2-R 750 triple, the same carb needed a 260-270 main jet. And on the 350cc cylinder of the F5 race engine, the main jet size went down slightly again, to 260.

Why? On a constant-size carburetor, the smaller the cylinder, the weaker the intake suction signal, and the smaller the fuel flow. Therefore, to give the 167cc cylinder the fuel it needs, a larger main jet must be used than for the more strongly pulsing 250cc cylinder. Flow through the carburetor is not steady and continuous, but is a series of pulses.

Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.

 

Like I said; general rule of thumb. There are exceptions.

Consider that the intake pulses of a 2-stroke engine happen twice as often, and are of a different duration and strength. Additionally there is more unburned fuel present in the blowback from the intake ports closing (as well as in the exhaust), so scavenging that unburnt fuel (desirable) plays a role in jetting choices (more fuel to scavenge = smaller jets needed to get the same AFR).
Also consider that the jetting needed for maximum power production in a race setting (midrange to max RMP (rideability at off idle can be sacrificed for more top-end), no concern for emissions or engine longevity beyond the racing season (if you have the money)) is going to be different that what is typically needed for a street setting (Off idle to midrange (typically), emissions of great concern, and longvity of at least a day past the end of the warranty period (ideally far longer in order to build and maintain a reputation)).

No matter what, the AFR needs to be measured (plug chops, colortune, gas analyzer) and the jetting changed accordingly (and not just at idle).

Engines loooove to run lean. More complete combustion occurs on the lean side, which means more heat, which means a healthy-sounding engine...until it's lean enough that the pistons get holes in them, (you'll hear a lean backfire (popping) on deceleration long before you get to that point...if you listen).