93 Sport - New carb, new issues

[laddski]

I purchased a 93 Sport in July of this year. Did an immediate trip to Lake Powell and although it ran great the whole time, it was lacking on power. It has the pro boss, 285hp motor with the pro-tec. I discovered when I got home that the second two barrels were not opening up as they had rusted shut. With the old carb, I could hit full throttle and it would accelerate up to about 40mph without a problem, it just didn't have the "power" I was used to on my 93 Sport I had sold a year ago and that's how I knew there was a problem.

The carb was not worth rebuilding due to the corrosion in it so I bought a rebuilt carb from National Carburetors. I installed it with a new gasket directly under the carb but not under the riser and it started right up and idles great. However, if I punch it, it bogs down. We took it on the lake yesterday to see if it would do the same under load, and it did. I can nurse it through the mid-range rpms and then punch it and it has excellent power but it dies if I punch it from idle or close to it.

Looking down the carb when I punch it shows a nice solid spray of gas. Any thoughts? There isn't a lot to adjust on these holley carbs. The two mixture screws which I took to best lean idle and what I think is called the accelerator pump, which is the spring loaded adjustment bolt that you tighten or loosen. (Not the idle screw). It didn't seem to change anything when I adjusted it. It was as loose at it would go upon arrival and I tightened it to the point where the spring was almost fully compressed at idle and it didn't change the bogging. Has nice smooth idle at 900 rpm and no problem from 3000 on up.

[jdmoore59]

First and foremost, check for vaccuum leaks. If there are any open vaccuum ports on the carb, they need to be attached to something, or plugged.
Second, check that the choke is operating properly. It should be wide open once the engine is at operating temperature.

Other than that....


I bought a rebuilt carb from National Carburetors.

Does the rebuilt carb have the same size primary jets as the original? Swapping out the jets from the old carb to the new one may solve your problem.

The problem you are experiencing is pretty common scenario in carb. engines. These engines rely on engine vaccuum to draw fuel through the carb jets and idle circuits to make the engine run. When you "punch it" the engine vaccuum falls off the chart (causing a lean mix condition) and the engine has to rely on another mechanism to supply fuel during the low vaccuum condition. This is supplied through the accelerator pump. the accelerator pump blasts a fuel charge directly into the carb venturi for the engine to use while vaccuum and engine rpm increases sufficiently to sustain combustion. The accelerator pump is adjustable to an extent to allow for the appropriate amount and duration of fuel delivery to overcome the lean condition. Ideally the amount and duration of accelerator pump is just enough to overcome the lean condition during low vaccuum. This becomes a bit of a balancing act within the carb circuits. The size of the primary jets, the adjustment of the idle circuits, the duration and amount of the accelerator pump and the vaccuum pull actuator on the secondary system all work together to maintain a dynamic optimum mix. If you have the idle circuit too lean, it takes too much fuel from the accelerator pump to move from idle to under power and the accelerator pump can not compensate for it. It's better to be a bit rich on the idle circuit than too lean. If the primary metering jets are too lean, the engine will seem to starve a bit at mid throttle. If the engine staggers between mid throttle and wide open or has a momentary stumble as the secondaries kick in then you probably have the wrong spring in the secondary vaccum actuator assembly. Carb tuning can be quite a pain in the rear but it's all a part of "dialing it in" to optimum performance.

I hope I didn't get too much of this wrong or confuse anyone.

[laddski]

Response was very helpful. As a starting point, I am going to replace the gasket under the riser as it is suspect and likely 18 years old. I tossed my old carb due to the horrible shape it was in so I may have to play with the jets a bit if it doesn't turn out to be a vacuum issue. Thanks for your input. It will give me a good reference point.

[DanielC]

A good tool to use to help you tune a carburetor is a vacuum gauge.
This is a good page to help you interpret what the the vacuum gauge is indicating.
http://www.secondchancegarage.com/public/186.cfm

[jdmoore59]

That's an awsome article on engine vaccuum readings. Thanks for sharing.

[laddski]

Thanks DanielC. That is amazing and I had no idea even though I have been working on engines as a hobbyist for 20+ years. Thanks much. I will post what I find.

[laddski]

Time to update. I hooked the vacuum guage up to the motor Saturday and here is what I got. After warmed up, vacuum sits at a bumpy 17-18, but it is not a surging bumpy nor is it the ticking of a hipo engine. Fast acceleration, if I can keep it running, will drop to 5 and then back to 22 at 3k rpm. Vacuum is steady when at 3k rpm. Upon fast decel, it will only go to about 24 and then drop back to normal. According to the link, it sounds like maybe rings but I think there is still something else causing the bogging. Rings alone shouldn't cause the issues I am having.

Will the GT heads provide a reading of 20 or are they considered high performance and won't read that high?

My next step is to check compression on each cylinder.

[laddski]

Another update. 1.5 years later and still a problem. Haven't had the boat out due to life issues until this past week at Lake Powell. Boat was useful for idling in the no wake zone and skiing at 36mph. Nothing in between.

I have played extensively with the shooters or nozzle, cam, accelerator pump, cleaned and checked jets and a bunch of other stuff. Nothing changes my problem. The boat starts great, idles great, goes into gear and gives me wakeless great and I can usually punch it and with some heavy pumping coaxing, it will then accelerate fast and hard up to 35+ mph and cruise there fine. I was able to ski thankfully but not much else. It simply will not run in the mid-range at all. Bogs and dies. If I decelerate from 35mph it will cut out as soon as I hit about 33mph or roughly 3500 rpm or less unless I just drop it back to idle.

Anyone have any new ideas? I don't think it is the accelerator pump circuit because I should be able to come down from 4000 rpm and run since it won't be using the accelerator pump at that point. Jets are clean.

[fb_rider]

Do you know anyone with a known good carb from a boat with the same engine? If so, see if they'll let you switch it out to rule out the carb. I had an older Nautique with a carb that was driving me crazy no matter what I did to it. It ran great with my buddy's carb on it. If that's the case, get a carb designed and jetted specifically for your engine.

[Mamigacz]

This might be related to ignition timing.

-Make sure your inital timing is around 8 degree's before top dead center (BTDC).
-Make sure all your mechanical advance is in before 3000 RPM.
-Make sure the full mechancial advance is 34 degrees BTDC.

I can give you a proceedure for checking full mechanical advance if you don't know how to do it.

[Mamigacz]

If the timing checks out, here is what I would do with the carb.

-remove the primary bowl (the gasket might rip)
-remove the metering block (the gasket might rip)


-See pic1. spray carb cleaner into the holes that the pencils are in. Carb cleaner should spray out the air bleeds on top of the carb. If no carb cleaner comes out, an air bleed is clogged. If its clogged, you found a problem.

-See pic2. Make sure to remove the jets and inspect if they are clean. Spray carb cleaner in the holes that the pencils are in. Take note on the where carb cleaner spray out.

-See pic3. spray carb cleaner in the holes that the pencils are in. Take note where the carb cleaner sprays out.

-See pic4. This is view of the transition slots. This is part of the idle circuit. It adds more fuel when coming off idle. Make sure there is nothing stuck in these slots. A paper clip will help to probe inside.


Because your engine is idling fine, my suggestions above don't include anything related to the idle circuit (with the acception of the transiition slots). If you see issues with carb cleaner making its way out of a passages, something is clogged. I know its a new carb, but its not out of the question that a chip of metal something else got stuck in one of the passages during the machining processes.

[laddski]

Final Update - Problem is fixed! I finally got tired of messing with this rebuilt carb from National Carburetors and bought a new Holley 4160 80319. This carb is specifically built for this motor out of the box. This is what my rebuilt carb from National was supposed to be but there are a number of differences now that I have my new one. Slapped the new carb on and it runs great. Checked timing and I was at 20 deg at idle. Adjusted to 8 deg and 34 deg max. All is well. Runs like it is supposed to.

Thanks to all that made suggestions. Particularly on the timing. Don't get to take it out for another 2 weeks but I can wait.