16-12-2016, 08:29 PM #1
750 quick fuel carb 308 tuning question
HI guy had eng built and eng dynoed, quick run down 308 288 cam alloy heads harrop hise 2'' spacer 750qf carb.
right heres my question for a carb expert or i might be barking up wrong tree. motor made 460hp
starts and runs from cold ok better when warmed up, drive it seems good pulls hard doesnt miss, flat spot or anything silly but when i turn it off it will not fire up again. it just turns over an over to the point of battery going flat and then it will just cough enough to give it couple pumps on accelerator and it might fire up. so i thought i was not get spark or 12v when cranking, its running a ready to run msd and a blaster3 coil no box. i was getting voltage drop at coil so i ran a relay straight to coil from battery. didnt help. optimum 750cca battery
so carby from what iv done, has 2.5pv 77air bleeds and std jetting from box, carb is brand new. im lost......
16-12-2016, 08:43 PM #2
Found the engine harness plug was brittle on my vh causing a voltage drop or sometimes really poor connection acc power to coil that it wouldn't start.Most times it was good then 1 out of 10 starts it played up like it was just about to fire but wouldn't.
Other thing didnt help was 55amp alternator at night....
Usually when cold just 2 small pumps,crank it...when it fires just dab throttle untill gets some heat or idles by itself.
After that usually just a few cranks it fires then dab throttle untill it idles on it's own.
Last edited by gtrboyy; 16-12-2016 at 08:50 PM.
16-12-2016, 08:49 PM #3
16-12-2016, 09:05 PM #4
- Join Date
- May 2013
- Shit Creek
Try foot flat next time it will be flooded...
I have a qft650 on mine (blow through)i have just got around to purchasing a phenolic spacer.
PHENOLIC CARBY SPACER 2" 4 HOLE - RPCR9135 | eBay
Mine boils the fuel out of the bowls
16-12-2016, 09:10 PM #5
Make sure you have good power to the ignition first.
What fuel pressure do you have?
Sight plugs for fuel bowls fitted? where are they at?
2.5 Pv, really? Not liking that, but that is not your problem here.
16-12-2016, 10:36 PM #6
- Join Date
- Feb 2013
- lurking in the shadows
Normally too much initial timing.
There wont be many street holdens with 750's that need less than 8.5" pv, hell, my shitter engine with firstname.lastname@example.org needs a 10.5.
16-12-2016, 11:09 PM #7
- Join Date
- Jul 2009
- Sometimes Melbourne
What lash settings you running ?
17-12-2016, 01:12 AM #8
17-12-2016, 01:14 AM #9
17-12-2016, 01:22 AM #10
Is there any other why to wire up dizzy. Im using original vk pink thick wire as an ign trigger to a relay an all new wires str8 to battery an coil.
17-12-2016, 10:54 AM #11
Might be a case of low air speed thru the intake and heads on wind up , has
it got a reduction starter or is it winding over fast ?
Last edited by 3554me; 17-12-2016 at 10:57 AM.email@example.com on e85
17-12-2016, 11:17 AM #12
Unless there is a major issue with the carb the power valve will have no effect on the idle circuit, the whole "set your power valve at half your idle vac" is a myth. My old combo had 5" vac at idle in gear and I still needed to run a 6.5 power valve to get the AFR's right, you need to check your vac at cruise and see what it drops off to when you accelerate. If yours drives nice with the 2.5 it's likely over jetted to compensate, the power valve wont be opening till it's near full throttle. The power valve is part of the main circuit which has no effect at idle.
With your hot start issue, have you tried giving the throttle a pump or two before cranking? You shouldn't need to do that but it will show if it's maybe a little lean when starting.
17-12-2016, 11:35 AM #13
I'd go with making sure there's good power to the ignition.. i had the exact same issue at one time.. once its warm wouldnt fire. I was pulling my hair out ****ing around with the carb turns out it was a bad ignition earth. fired with a turn of the key after that.
17-12-2016, 01:52 PM #14
17-12-2016, 03:34 PM #15
Pink wire you're using comes off ignition switch block but you're using as trigger & power off battery for the coil so that should be good.
Mine is crap vacuum too still fires & set up roughly same float levels etc
Think maybe yours tad too much initial timing?
ve sidi v6 vacuum booster will help get brakes back running that to booster although wondered if it helps or hinders if run to manifold.
Last edited by gtrboyy; 17-12-2016 at 03:38 PM. Reason: .
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