NEED A LITTLE GPU HELP

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GMAC 76
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NEED A LITTLE GPU HELP

Postby GMAC 76 » Thu Oct 29, 2015 3:40 pm

working on a 220 gpu...dual mag, mechanical fuel pump, 5 blade warp.....mags are fresh rebuilds good wires new plugs...timing is right on and mags are in sync.....new out of the box 500 holley carb....engine starts and idles perfect but when we power the engine up it cracks and pops and doesn't want to power up...let off the gas and everything is fine...stab the throttle and it accelerates fine for a few seconds but then starts the cracking and popping....I believe it is in the carb but I am not real familiar with the automotive setups ie. jets power valves squriters etc. ....any suggestion would sure be appreciated...thanks, greg

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Re: NEED A LITTLE GPU HELP

Postby GeeLeDouche » Thu Oct 29, 2015 3:50 pm

This may not be much help, but have you checked for intake or exhaust leaks? that's my first thought when I hear "pops" in a post.
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Re: NEED A LITTLE GPU HELP

Postby scottyroll » Thu Oct 29, 2015 3:53 pm

GeeLeDouche wrote:This may not be much help, but have you checked for intake or exhaust leaks? that's my first thought when I hear "pops" in a post.


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Re: NEED A LITTLE GPU HELP

Postby goldhunter_2 » Thu Oct 29, 2015 4:58 pm

well you know I just have to tell you just put a decent AC carb on there :violent1:


Check everything but only one thing at a time. you didn't say whether it had gpu cylinders or 520 conversion , Gpu cylinder that holley should have barley enough air but if memory serves me you had to change the jets (don't remember # now) if 520 cylinders your starving motor for air. A leakdown test will help check your intake an exhaust (bend valve, broken valve seat, broke spring, etc) check for intake leaks , also may sound stupid but double check you don't have any wires crossed
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Re: NEED A LITTLE GPU HELP

Postby GMAC 76 » Thu Oct 29, 2015 7:43 pm

It is a 220 gpu with the original gpu cyls machined for dual plugs....complete top end job by Schmidt aviation all new intake hoses and gaskets...new holley was put on right out of the box by owner...only adjustment was idle mixture and idle rpm.... has excellent throttle response then the turkey shoot starts

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Re: NEED A LITTLE GPU HELP

Postby chugalug » Thu Oct 29, 2015 7:47 pm

Sounds to me like it's leaning out

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Re: NEED A LITTLE GPU HELP

Postby Chris gpu » Thu Oct 29, 2015 8:21 pm

I have a stock single mag GPU and run a 2 barrel Holley 500. I bought mine from the local parts store and didn't have to change any jets. It runs awesome.

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Re: NEED A LITTLE GPU HELP

Postby Chris gpu » Thu Oct 29, 2015 8:25 pm

Also from my experience I have had brand new plugs foul and do the same thing. Idles fine but when you get on the throttle it starts backfiring and popping

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Re: NEED A LITTLE GPU HELP

Postby midnight rider » Thu Oct 29, 2015 8:41 pm

Check the power valve in the carb. Had the same issue years ago. One bad backfire from being barely out of time blew the power valve. Had us chasin ghosts for two days. Lol.

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Re: NEED A LITTLE GPU HELP

Postby GCRedfisher » Thu Oct 29, 2015 9:40 pm

midnight rider wrote:Check the power valve in the carb. Had the same issue years ago. One bad backfire from being barely out of time blew the power valve. Had us chasin ghosts for two days. Lol.


If it was a New Holley, it most likely has power valve blow out protection, my new 750 DP has it and according to Holley the 500 cfms have it also
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Re: NEED A LITTLE GPU HELP

Postby dodobird » Thu Oct 29, 2015 9:51 pm

ive seen where little bits of packing material gets in the fuel line and fuel inlet on carbs right out of the box my demon had little white pieces of something in the bowls it was soft like foam :florida:
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Re: NEED A LITTLE GPU HELP

Postby swampfox » Thu Oct 29, 2015 10:21 pm

I know mags r ur thing. With that said have u checked to see if the is any cracks in the plastic block.on the inside or if maybe something inside is causing it to fire across itself. Had that problem.With my 220. Would idle fine, but would pop under load. Hope this helps.
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NEED A LITTLE GPU HELP

Postby Comanche-pup » Thu Oct 29, 2015 10:53 pm

You are not getting enough fuel in your bowl, adjust your float, fuel should be level with the screw on side bowl. When float is adjusted too low, idles fine but when you try to gouge it, or WOT you run the bowl dry


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Re: NEED A LITTLE GPU HELP

Postby GMAC 76 » Fri Oct 30, 2015 10:52 am

got a nice list of great ideas from you guys and when I get the boat back to the shop I will try everything till I get it right....really appreciate all the help

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Re: NEED A LITTLE GPU HELP

Postby John Fenner » Fri Oct 30, 2015 12:06 pm

Do a mag drop check as one mag could be off, see if it does it on one mag at a time.
I never finish anyth,,,.

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Re: NEED A LITTLE GPU HELP

Postby glades cat » Fri Oct 30, 2015 12:08 pm

I believe the Holley 500cfm comes with #72 jets and 50cc pump. I remember changing mine to 74's. Check float level. Remove the bowl and metering block and check for debris.
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Re: NEED A LITTLE GPU HELP

Postby GMAC 76 » Fri Oct 30, 2015 2:31 pm

John Fenner wrote:Do a mag drop check as one mag could be off, see if it does it on one mag at a time.

mag drop at 1800 rpm is about 75 rpms....I did try one mag at a time and the results are the same that is why I am thinking something in the fuel delivery...it runs really well until you ask it for wot......I have also been told that the carb has a ceramic filter at the inlet and should be removed to free up fuel flow into the bowl I assume it is still in there cause carb is out of the box installed

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Re: NEED A LITTLE GPU HELP

Postby John Fenner » Fri Oct 30, 2015 5:52 pm

Bingo!!
I never finish anyth,,,.

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Re: NEED A LITTLE GPU HELP

Postby GMAC 76 » Sat Oct 31, 2015 3:09 pm

when I get the boat gonna take a second look at the plug wires as possible problem.....found out today they are 12+ years old........

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Re: NEED A LITTLE GPU HELP

Postby swampfox » Sat Oct 31, 2015 4:14 pm

Who wired it. I would make sure someone didn't try to put power to mag. Just a thought. Maybe they didn't know it always fires and u just ground it out. Had a buddy who did that and I believe it was the condensor that was making his boat do that exact thing. Idle fine, then pop and crack under power. Hope this helps
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Re: NEED A LITTLE GPU HELP

Postby GMAC 76 » Mon Nov 02, 2015 8:05 pm

got the engine back and pulled the spark plugs....they were solid black (not oil)....took carb off and when I pulled the bowl off one of the jets fell out on the bench...guess I found the problem...

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Re: NEED A LITTLE GPU HELP

Postby mojoe » Tue Nov 03, 2015 1:29 pm

Not skrewed in?

Echo en Mexico?
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Re: NEED A LITTLE GPU HELP

Postby GMAC 76 » Tue Nov 03, 2015 2:29 pm

mojoe wrote:Not skrewed in?

Echo en Mexico?

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Re: NEED A LITTLE GPU HELP

Postby Comanche-pup » Tue Nov 03, 2015 3:11 pm

^^^^^^^^
Thats too funny


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GeeLeDouche
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Re: NEED A LITTLE GPU HELP

Postby GeeLeDouche » Tue Nov 03, 2015 5:30 pm

hahahaha.. Im glad you were able to figure it out without to much trouble.
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