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Discussion Starter · #1 ·
Just pulled the carb apart after having it tuned by a shop a few years ago.

I am removing the power valve & need to know what to chang the jetting to (How much to jet up?????) I thought I was going to run it squared but its already there with the power valve????

Current jetting is 85 squared. Car on motor has a very bad bog if I try to footbrake it. Its almost as if it shuts off then runs like hell!!!. Before the car was "Tuned" it would sixty 1.51 on motor, after tuning it was in the 1.7's but picked up 3 mph in the 1/4. Car runs 11.14 @125 on motor & 9.55 @ 140 on a 225hp shot on a Big Shot plate. Motor is a 331 sbf.. 10 to 1 comp, AFR 205's, 750HP, C4, 3800 stall nitrous converter, 4.30 gears. Car weighs 3200 loaded. Timing is at 36 total & I pull 9 deg for the n2o. NGK 9's gapped at .030"

Any Ideas on the jetting??? I want to get this thing up to full potention but dont know "much" about carbs. Could use all the help I can get before stepping up to a fogger & plate. I have the fogger but want to get the carb right before going crazy & having a boat anchor.

I dont run it on motor for the most part its a nitrous only car..
I know the cars has more in it & I'm leaving alot on the table with the shitty tune.

Thanks!!
 

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Well, I'll take a stab at it. I haven't touched a carb except for my weed whacker and lawn mower since 1987 but here is what I have seen: Too much pump shot!

You say you are running good once it gets over the inital cough. You also said that you picked-up 3 mph in the 1/4, well, that is a lot. The carb must be very close to what your motor needs now. Maybe someone put really big accel pumps or squirters in there previously to cover-up the lack of jets before.

Was this carb ever on a street car and they were trying to get good mileage out of it before with small jets? A lot of people will do that then put HUGE accel pump cams in there to try and make it work.

I never saw any of my carb buddies run square jetting WITH a power valve. I think someone simply screwed-up before and was trying to fix a different problem.

One of your best-bets is to put a wideband on it like an LM-1 with the RPM Convertor and record some motor-only passes and verify that it is going pig-rich at the hit. Get it tuned on motor before you step-up the nitrous. Even with a tight N2O convertor the A/F shouldn't take such a dive like that.
 

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Well on a 406 street strip motor,I have a proform 850 and have 3.5PV in front and back is blocked off ,jetting is 82 front and 88 rear and it seems to be liking it.
 

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Discussion Starter · #4 ·
Thanks for the replies, I must have been drunk when I pulled it apart last night..... It actually had 75 & 85's so that makes more sense......I've been told the power valve is worth 10 jet sizes so I'm going to rebuild it tonight & going to square it up at 85 without the power valve & make a 1/8 mile run & see how things look. Then we will get back to the n2o;)

Thanks!!
 

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Normally

Yes, under normal circumstances 10 jet sizes is a good starting point. That's the approximate jet value of the power valve.
 

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"Darkness"
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Thanks for the replies, I must have been drunk when I pulled it apart last night..... It actually had 75 & 85's so that makes more sense......I've been told the power valve is worth 10 jet sizes so I'm going to rebuild it tonight & going to square it up at 85 without the power valve & make a 1/8 mile run & see how things look. Then we will get back to the n2o;)

Thanks!!
Good thinking. Make sure it has jet extensions in the rear while its apart.
 

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Discussion Starter · #7 ·
Has jet extensions in the rear... Ended up with 84 front & 85 rear, didnt have another set of 85's... I'll post up after I take it out Sat.. Wish me luck as this was my first carb rebuild :rolleyes:... Wasent as bad as I thought it was going to be, definatly not intimated by a carb anymore.. Wish I would have torn it apart earlier..haha:p

Now I just need to learn these damn air bleeds!! Thats how they got the mph but killed the sixty foot.. Works on n2o so I guess I can't complain. Just sucks when cruzing & someone tries to go from a roll & the engine shuts down then blows the tires off as soon as I mash the gas.. Guess I cant have the best of both worlds. :( .. Also cant footbrake the car.. Maybe this will help.. We'll see!!


I got the carb new & is basically stock Holley HP... Didnt have the bog before the tune so not sure whats going on there... The only change during the tune was the air bleeds... They are 75 outside & 40 inside for the fronts & 70 outside & 41 inside for the rears.. Does that make sense??? I would think they would be the same inside & outside.. I understand them being different front to back just not inside to outside, maybe theres something to look at there??

Thanks again for the help!!
 

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I run a 750hp on a 357 and have no problems with it.That bad bog is probably a lean condition.I am running 75pjet and a 76sjet with both power valves in at #2.5.I think going to 85's are a big jump.I would go back to the #35 hsab and see how it does.What color pump cams are in there?and what position are they in?Ideally you want the "inner"hsab squared front to back.Just for clarification,The outside airbleeds are for the idle circuit and the inners are for wot curve.Putting the #35 back in will richen up the fuel curve at wot.You may be able to clear the bog up by making a pump cam change.The car is flying the way it is just clen up the carb a little and you should be good to go.What the shop did was fatten up the secondary side of the carb,but then they leaned it back out by putting the larger airbleeds in.If I was you,I would go back to 75pj and 75sj with the pv in and change the hsab's first.See how it does,If the bog improves put some different pump cams in and play with that a little.

Scott
 

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just to give you a simple easy analogy to the air bleeds,take a glass of water and a straw.Now,drink out of the straw,you will have a full column of water.Now poke a hole in the straw,Now you get less water and you also get air.Poke a larger hole in the straw and you get even more air and even less water.That is probably the easiest way to understand the air bleeds.
 

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what size pump is it? 20cc, 30, or 50cc?
make sure it adjusted propperly, .015 clearance first.
also I would try diff squriter to start, you may need to increase pump vol. if your using the smaller of the three pump's.

it's been my exp. that a boog or hesatation is brought on by a momontary lack of fuel .
although this may or may not be your problem it would be the first place I'd start.
holley offer's a trick kit with all (but the pump)part's to do this.
Matt.
 

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did'nt see that you said what carb this is ( may have missed it) but is the front the same as the rear?
got a part no. for the carb?
if I recall most have a 30cc on the rear is the front the same?
 

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Discussion Starter · #15 ·
750 HP, 4150
Has 30cc front & back with .025" squirters & after looking at the cams they are def pink.(I thought they were just a faded red but after looking at the diferent cams they are def pink.) They have 3 position holes & are in the first hole.

Thanks for all the help.. I know I should have dug into this thing a long time ago.
 

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are the squirter's the solid type or the one's with small tube's, you can drill the solid type out to increase volume, .025 is a little small I'd start at .031.
I like the tube style squirter's they reach out into the carb a little more,.
you also may want to change the cam to one with a little more duration and a sharper ramp. (to get it going sooner).
good luck, Matt.
 

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Discussion Starter · #17 ·
Well...... Put the carb back on tonight & went for a spin.. Driveability was for SHIT!!! At any cruzing speed the car was bucking like a five speed.. BUT.. Under accelleration it ran GREAT!! I'm thinking to try & put the PV back in & jet it back down.

Is it normal for driveability to be that bad without the PV???? I really didnt expect it to be this bad. I run the car in street classes where it mandates a cruze before competition & this isnt going to work.

Worst case I'll be calling Patrick & ordering a carb. Hopefully I can make this thing work.
 

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Discussion Starter · #19 ·
Put a 3.5 PV back in it & cruzed it for about 1/2 hour (Orig was a 6.5) .. Almost back to normal driving wise!! Old PV was a 6.5 & with the 3.5 there was some & bucking (& I mean just a little) but tolerable & still cruzeable.

Think I'm still going to call Pro Systems Tuseday just to be done with it.

Thanks for all your help!!!! I appreciate it for sure, just dont have the patients to try to tune something I know nothing about. This is my first carb'd car & I'm def on a learning curve!! So I ......:Kneel
 

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if it's running good other than the "boog" I'm think'in not enough pump shot.
That's what i would think.... I've seen really rich pump shots make black smoke...never bog...that's a lean hole in the curve...

what front pump.... put the 50cc pump on the front if it's the small one.

mike
 
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