Holley 2 barrel on 3.0 GS, accelerator pump issue

[Glastron175]

Still fighting the hesitation problem. Decided to take my race car friend's advise and drill out the accelerator pump nozzle to .031 (currently .028). This should put a little more gas out when accelerator pump is advanced. But when I took off the carb last night and removed the pump nozzle, I noticed the little gasket under the nozzle had an inside diameter which was just barely large enough for the screw that goes thru it. It appears that it could be restricting the amount of gas that can be pumped up thru the nozzle. So I searched thru my carb kit, which has myriads of gaskets that were not needed for my carb rebuild, and found a gasket that fits and has a larger inside diameter. Perhaps it is the one that was supposed to have gone on it to begin with. So I put it all together and backon the boat. Tonight I'll put it on the muffs, fire it up, and see if the hesitation problem has changed. If not, then I'll go back again and drill out the nozzles. Will let you all know how it goes.

 

[Glastron175]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

Well I tested it this evening on muffs and could not tell much difference. However, I decided to try once more adjusting the mixture screws before resorting to drillling out the accelerator pump nozzles. I adjusted them much more rich, about 1 3/4 furns out, and it does seem to be better. This is not the best idle but if it solves the hesitation problem, I'll go with it. Hope to get it on the lake tomorrow to see if it still performs better then. I don't know why the idle mixture affects performance at higer RPMs, but it seems to. I can now advance the throttle quickly from about 2000 RPM without the hesitation I was getting before. I thought the idle mixture screws only affected the idle because the jets take over as the RPM is advanced. Any thoughts on this???

 

[Maclin]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

The idle mix also affects transition to the main jet flow.

The gasket is one of the "tougher" material ones, and will be cone shaped once it is used, there should be a check ball under the screw. There is also a check ball in the pump housing. Both need to be in place.

The amount of fuel pumped is determined by the accelerator pump housing capacity (35cc or 50cc). The same amount of fuel is pumped each time. Nozzle size affects the duration of the pump shot. Larger nozzles let more of the fuel come in sooner, smaller nozzles stretch the shot out some. Some engines want a lot all at once while some need a longer shot to get to where the main jets are flowing. The pump lever cam also plays a role on the pump shot.

All that said, the OEM specs for that carb are more than likely just right, so be careful when changing the squirters. I would buy them and not drill any out. They should be available at Holley performance shops.

 

[Glastron175]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

Thanks for your reply. It is obvious that you are an expert in Holley carbs. If the mixture adjustment that I now have does not solve the problem, then I may also try changing the cam position to no. 1 instead of no. 2 position. That will pump more gas earlier as I understand it. However, it currently is in the position that the specs call for.
I plan to keep foolin' with it until it accelerates promptly as it should, without any hesitation. I will report on how it goes, maybe someone else will benefit from it.

 

[Maclin]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

The alternate cam positioning hole is to engage the pump lever later in the throttle travel. It is there for engines that need the throttle idle position to be slightly open due to large cam overlap, mainly for race type conditions where idle RPM's may be 1200-1500 rpm. Right now I cannot remember the numbering if #1 is for more stock engines or #2. When it is in the one designed for race type engines the pump lever will not begin to move until the throttle plates are open somewhat.

Good luck, keep at it and you will get it working well I am sure.

 

[Glastron175]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

Thanks Maclin. Since you know so much about Holley carbs, you may be able to answer something else I have wondered about. On the Holley web site talking about the power valves, they say that the size chosen depends on the idle vacuum. The suggestion was to measure vacuum at idle, and choose a power valve of 1/2 the value measured. For instance, if vacuum at idle is 13, then chose a 6.5 power valve. My power valve is a 2.5 value. However when I checked my vacuum at idle it measure 15 inches, meaning I should have a 7.5 power valve. Is there a difference between marine and auto carbs to explain this?

 

[Maclin]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

The lower number power valve specs are due to the duty cycle of a boat engine compared to an automotive application. Boat engines are under near constant load (lower vacuum) even when cruising, different than in an automotive application where crusing typically has lower loads and relatively higher vacuum. The power valve number represents how much vacumm to keep it closed, so engines with lower vacuum need lower numbered power valves so they will stay closed until needed during acceleration. Holley's for boat engines typically run 2.5 to 3.5 power valves. Any higher and they tend to stay open, constantly enriching the fuel flow causing an over-rich condition.

 

[Glastron175]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

I wasn't able to get on the lake today to see if adjusting the mixture more rich helps my hesitation problem. Probably will be the end of this coming week before I can get out there. Will update this thread at that time.

 

[Glastron175]

Re: Holley 2 barrel on 3.0 GS, accelerator pump issue

I got it back on the lake this morning, and to my surprise, the hesitation problem is practically gone. There is still a minor hesitation when I advance the throttle really quickly, but it doesn't die like before. If I advance more slowly but smoothly, it does great. Runs at every RPM very smoothly too. So, here is my finding. I have the mixture screws set about 1/2 gto 3/4 turn more rich than what gives the peak RPM at idle. I can understand how it could affect the transition from idle to WOT, but still don't know why it solves the hesitation problem from 1/2 throttle to WOT. Any comments?