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Discussion Starter · #1 ·
Hi All,
Just in the final stages of getting my self and race bike ('88 750J) back together after a lengthy hiatus and I'm hoping the collective gixxer wisdon will be able to help.
Engine now has:
1. wiseco 1mm oversize pistons, giving a measured 11.6:1 (bit short of the claimed 13:1 - fortunately!)
2. valve re-cut and port job by local gooroo, who raced Suzukis alongside such notables as Mick Doohan and Robbie Phyllis. Same individual dialled in the cams for me, and the level of respect he has locally leads me to believe that he got it perfect...
3. Reprofiled cams, and reinforced rockers.
4. Carbs are the original 38s, with a dynojet kit and 170 mains jets, which we are expecting to be a bit rich (It's not allowed on the dyno until it's done at least one day at the track without blowing up).
5. 4 into 1 system and an end can that presumably adds very little backpressure, since it doesn't seem to affect the amount of noise on or off.
6. Dyna coils and 2000 ignition set at 38 degrees.

The problem comes just as the throttle is opened slightly - there's a lot of backfiring through all 4 carbs and it will fire the vacuum blanks off, which was very amusing for about 10 minutes. It idles OK (well actually - it idles like an engine with a lumpy cam), and when the throttle is opened firmly, it picks up with no hesitation and revs nicely. I've wound the idle mixtures out as far as 5 turns, raised the fuel level in the bowls by 3mm, lifted the needles - all without curing it. Now the bit that worries me. When the guy dialled in the cams he commented that I was one tooth out, and he would have had to slot the sprockets a lot more if he had left them where they were. I opened the covers later to see for myself, and in fact I had set it correctly, he has the intake cam advanced by one tooth. Again, this guy knows what he's doing, but I'm a little suspicious that the intake cams are opening too early, causing the backfires.
Has anybody got a feel for the baseline setting I should have on the carbs, and any experience in setting up reprofiled cams on these things?
thanks in advance,
alocky
 

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Discussion Starter · #3 ·
Thanks sickpup - you're right, it's got 36s not 38s. I can get my hands on a set of 38s, but I was under the impression that the 36s should be enough - (feel free to advise!). I'll see what the guy has to say about his cam timing tomorrow.
cheers,
alocky
 

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36 carbs are fine. As I mentioned before, my old 85 engine with 89 carbs and airbox + Hindle pipe was puting out 103 or 104 rwhp. I talked to a buddy of mine, who knew the guy that bought the bike after me and he said that he dynoed it again and it was at 107 hp. Insane??? I don't know, maybe the 2nd dyno was a bit off but the first number was right.
You have 170 jets in it? Isn't that too big? I'll call my friend on Monday to see if that is right. I don't know much about jetting and carbs but that number for some reason doesn't sit with me...
 

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try somewhere around a 145 main and see how that affects things
 

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Discussion Starter · #7 ·
try somewhere around a 145 main and see how that affects things
I started at a 144 and it was definitely a bit lean on full noise. It runs without any surging at full throttle with the 170, the backfire only shows up at tiny throttle openings around 2000-3000rpm with no load on the motor.
Today I spoke to the local carb/dyno dude, the head gooroo, and the guys who reprofiled the cam. The head guy said to throw the carbs away and buy a set of flatslides, the carb/dyno dude didn't see how that would solve the problem, and the camshaft engineers said there was no way the cam would time at one tooth out after they'd worked it. So I guess I'll break out the dial guages and degree wheel and set the cams to equal overlap myself. Bit annoying really, and I certainly don't want to buy a new set of flatslides, especially if it doesn't solve the problem!
Thanks for all your help so far...
cheers,
alocky.
 

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If the backfireing is just off idle I'd be looking at the idle jet. Sounds too rich. Lean it down a bit.

If that doesn't work try dropping needles a littlle.

Also check ignition timing.
 

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does it backfire in the upper RPMs 8k+ w/ the throttle closed? the kinda backfire u wanna listen for .. is the snap crackle pop backfire.. not the BANG kind...

basically... get her in 1st or 2nd gear.. run it up to redline.. then close the throttle... and listen..

if so, you're lean on the pilot circuit.. adjust screws or pilot jet to compensate..
 

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Discussion Starter · #11 ·
does it backfire in the upper RPMs 8k+ w/ the throttle closed? the kinda backfire u wanna listen for .. is the snap crackle pop backfire.. not the BANG kind...
Nope - only backfire is through the carbs at about 2000 rpm with the throttle slightly cracked. I've had a chat to the guys who re-profiled the cam and they tell me there is no way the timing could be shifted by a tooth. Tomorrow I shall break out the degree wheel and dial guage and set the bloody thing up myself.
I'll report back when I 've introduced Mr Valve to Ms Piston at 12000 rpm...
cheers,
alocky.
 
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