getting there with this carb/overrevving issue

Thanks to LoDollarDave, I took a deeper look at my carbs. Well, I don't have a 650 carb on a 750 engine, but it's not so simple.

My frame begins 5G8 010 xxx
Engine is stamped by the clutch bracket 5G8 - that's it, nothing else.

So according to Haynes, that's a 1981 750RJ, is that right ?

So I should have Main jet 120, Pilot 40, Jet needle Y13.

But I've got Mains of 118, (specified nowhere in my Haynes)
Pilots of 43, and needles of Y14 (both 750UK spec)

Is all that enough to account for the overrevving-at-idle problem ?

Thanks guys - I hope we're making progress, or at least that someone else will benefit from all this in some way :roll:

 

Pilot size sounds right - my 82 750RJ came stock with 43s. Mains seem a little small, but this shouldn't have a huge effect at idle - same with needle size. Can't help with the frame/engine numbers, but there's usually at least 3 more digits stamped on the engine case. Are you running stock intake and exhaust components? My bike first started 'zooming' (and no, not in the good way) when I switched to a K&N filter (airbox mounted) air filter. Apparently these beasties are sensitive with regard to intake configuration...

 

5G8 - you've got a XJ 750D (Fuel Injection) Japanese market only import.

I got one of these (1982) with carbs on - Main Jet 92.5, Pilot 40, Unknown needle.
Have got it running (a bit lean after removing the oxygen sensor) and have had split rubber intakes, vacuum leaks, various carb problems.

Am currently awaiting some info from my local bike shop re: proper jet sizes, so that it will run smoothly (lumpy as hell at the mo).

 

Hi Ade

Yeah the Jap market thing rings a bell.

But do you have just the 5G8 stamped on the case ? Or a full serial number ?

Would love to hear how you get on with those specs, as I'm a mech novice in any case. I have no idea what Fuel Injection means here, but that's what I've got the search button for.

Lo, yeah I'm stock air filter with Motad 4 into 1.

 

Hi,
5G8 - 020xxx is my frame number, and yes 5G8 is on the engine - apparently these Jap only bikes only have the 3 digits on the crank case.

Mine is also Motad 4-1 with stock air filter.

http://members.tripod.com/dave_jack/id27.htm shows XJ750D Fuel Injection in more detail and

http://home.netcom.com/~rvtucker/Seca.html has more info on the 650 Turbo which the 750D uses for the frame and other bits.

BTW, being one of the earliest fuel injection models - there are very few left with a working injection system - most have been carbed by now (also near impossible to get replacements).

Is it running rich or lean (black sooty spark plugs=rich)
Overrevving at idle - I assume you've checked the idle screw.
Possible air/vacuum leak on rubbers from carb to engine.

 

Ade,

It's worth running a search here on '5G8' - there was a guy who found a bike with the exact same engine markings - anyway, someone said it was a fuel injection, someone else said it's not, it's just a jap market bike ie 750A.

Then again my frame number is outside the range given for the 750A...

Will you let us know what that bike shop tells you ? Cheers

As for running - it is a bit lean, the overrevving in neutral when hot is horrendous (starts up ok) - have checked all rubbers, ok, and fitted new butterfly shaft seals. Carb thoroughly cleaned, but with non-spec jets (see my post of yesterday).

Good luck with yours

 

Probably the 4-1 with stock filtration that's giving you the most grief right now - tough to balance a mismatch. You may want to try a more free flowing filter. The K&N airbox filter is not too bad if you don't want to go to pods right now.

Once you go to the less restrictive intake, you're gonna be even leaner. You may want to pick up an extra set of idle jets and open them up with a #77 drill bit to yield approx. #46. If it still zooms at idle at 3+ turns open on the mixture screws, then you may have to go even larger (I'm at about #50 right now, but mine stopped zooming at about 46 - the rest is tune and tweak). You'll know you're close when you get no zooming at about 2.5 turns open on the mixture screw. If it bogs down or hesitates when you blip the throttle from idle/low throttle while riding, then you've gone too far and should downsize your idle jet.

Once the zooming is under control you should get your mains dialed in next - 118s sound crazy lean to me, but it sounds like you have a unique or at least unusual set-up. With a 4-1 and decent intake breathing I would think you'd be in the 130-136 range. See the factorypro website for CV carb tuning, their procedure worked well for me and the step-by-step instructions and tips make it pretty straight forward.

If your sick of playing with carbs already, then a switch to a more restrictive 4-2 or stock type exhaust is in order - to get your jetting right you're gonna be pulling and re-installing/synching/adjusting/test riding through at least another 3-4 iterations to get it right, but boy will it pay off in throttle response, idle, top end, and overall smoothess.

Sorry to all those who are sick of my long posts - I can be a bit of a windbag and I know it. If we're tying up to much forum space you can PM me.

Dave

 

Its better to be long-winded and COMPREHENSIVE -- than have someone else facing the same issue MISS something important. Sometimes adding what seems like a superflous comment saves somebody's butt that's trying the same thing.

 

Checked the XJCD and your 5G8 goes to the following:
XJ750A 1981 JAPAN
XJ750D 1983 FUEL INJECTED JAPAN
XJ750D-2 1983
XJ750E 1981

Yamaha could use a better system if you ask me.

 

Oops! Forgot to add that the frame# range on the XJ750A is from 010000 to 099999so that makes yours a 1981 XJ750A sold in Japan.

 

I wonder if carbs were off a 650?

 

Thanks Blue, that's a definite on the model I think.

Could be a 650 carb, but the jets don't tally with Haynes on this (I posted the #'s earlier on this thread).

The Motad would account for non-standard jets wouldn't it ? Or is there another angle on the 650/750 question ?

 

No I don't think they are off a 650. But anything is possible. A 650 with pods and a 4 into 1 maybe. I don't have any experience with re-jetting. Stock jets on a 650 are usually a main of 110 or 112 with a pilot of 40. If he is reading the jet from cylinder 2 or 3 he may find that the jets in 1 & 4 are 120's. Yamaha did this to keep the inner cylinders cooler.