GregLuce

I'm a new boat owner. I have an '83 PCM 351, and it will run up to 3800 rpm strong, but if I push past that up to 4K it feels like it's starved for air or fuel or something. It backs down to about 3500 on it's own. To get back to 3800 I have to ease off the throttle, then accelerate back up to 3800.

I cleaned the air filter element with mineral spirits and then soapy water. Today I'm replacing fuel filters. How do you measure fuel pressure? I'd rather not just buy a fuel pump.

One other issue. My idle is around 1100. Once after it was warmed up, it was idling around 800, but I can't repeat that. What's the best way to adjust idle? I think there's a cable adjust, and some other idle adjustments right?

TIA
Greg

Rodger

It sounds like the float resevoirs are emptying after running hard. When you slow back down they refill again. The fuel filter you are in the process of changing may fix the problem. I have seen screens at the inlet of Holley carbs. I typically remove them and rely on the main fuel filter, or add an in line filter. If you want to install a pressure guage, it should be as close to the carb as possible in the fuel line. Being that you have a 351 PCM, it sounds like you may have a mid engine inboard ski boat. If this is the case, you can remove the engine cover and watch the guage as the boat is running at full throttle. Any pressure above 3 or 4 PSI at WOT on a mild engine, such as yours, should be sufficient. As for the idle speed adjustment, since it idled at 800 previously, check for binding or stiffness somewhere in the cable or linkage. Can you push on the thottle lever at the carb and slow it down?

GregLuce

It sounds like you know your stuff, thanks. I'll see about getting a fuel pressure guage tonight.

As for the idle, I tried pushing on the throttle cable and it didn't budge. What's the chance of the choke not operating correctly? I seem to recall most boats you can pump the throttle in nuetral to disengage the choke when it's idling fast, then it settles down to a proper idle speed. Otherwise, what's the proper way to adjust the idle speed on the carb?

I found this book I'm going to order which might be helpful:
How To Power Tune
Your Boat
By R.J. Miller

Covers Holley marine carburetor tuning tips from idle to wide open throttle performance. Marine carburetor maintenance and starage tips are included. There's a marine troubleshooting guide, too. $3.95

http://www.holley.com/HiOctn/Sales/S.../SrvLtrtr.html

Rodger

You make a good point about the choke. It should have a mechanism to create an increase in idle speed when first started. After it warms up some it should idle slower. Check to see if the fast idle mechanism (cam) is still activated after it warms up. The base idle should be adjusted with the engine warm. There is an idle stop screw to dial it in to the desired RPM. There is also two idle jet screws to adjust, that will affect idle and slightly higher than idle RPM. These screws are on the side of the carb, near the primary float chamber. one screw on each side. It's nice to have a vacuum guage for this but I usually adjust the jets by ear. Turn them in until the engine starts to stumble or slow a bit. Then back them out until it speeds up, and further backing out won't change it at all. At the point where the engine reached a higher and stable RPM, maybe back out 1/4 turm more or so. You may have to go back and reset the base idle stop screw again and possibly tweak the jets again after that. If you can, wait until you recieve your book. It will probably have good instruction with illustrations too.

mcollinstn

Many marine carbs do NOT have a fast-idle cam (but it sounds like this one does).

I would personally remove it if it has one, because you can set the idle with the throttle lever to wherever you want it and it will stay (unlike a car which will return to the stop when you take your foot off the pedal).

TPCRacing

What Holley carburetor are we talking about here? Vacuum secondary or double pumper?

I would definitely look into the fuel pressure issue, check your filters, and or remove those screens as previously mentioned.

Your carburetor model all together might be hurting your performance. Give me some more info.

~Ty

GregLuce

Last night after replacing fuel filter I WAS able to shut off the fast idle with a quick pump on the throttle and it idled at 800. But after testing the high-speed operation the idle was back up to 1100 and I wasn't able to shut it down with the throttle.

There are 2 numbers on the carb body. On the outside there is 2R 4608B then after you remove the air cleaner on the top is another number: 6R 4920B.

I'll try removing the screens tonight.

Rodger

Your secondaries may not quite return to full closed position, causing the fast idle. Check and see if they are binding. Try to push, just that linkage, further closed.

TPCRacing

From the sounds of your last post, your throttlebody may be dirty causing a bind in the throttleplate to throttlebody relationship. This requires cleaning the bores which may be dirty from carbon build-up and possibly realigning the throttleplates. Sounds easy enough but it's not, whether over the phone or through e-mail.

Do you have alot of black carbon build-up on the inside of the bores? Whether or not it's a mech.secondary or vacuum carburertor, the secondary needs to completely reseat once off throttle. If it sticks then you could have a run-away RPM. Not good.

You might consider rebuilding this carb or having someone go through it.

Good luck. Call me if you need any assistance on this set-up 877.872.2272