5.7L Rochester carb issue we think

[mr300z87]

Hi all, I have posted before regarding my friends 5.7L in an 89 Sea Ray Sundancer 250 that ingested water on it second voyage after a rebuild. Anyway last fall he pickup a running drop in 260 HP 5.7L Rochester 4 barrel thunderbolt IV engine of 89-95 vintage. Before he bought it they did a comp check while he was with them and all 8 were between 140 and 150. I personally heard the engine run on the ground when we pick it up. We installed the engine last fall an put it away for the winter. Back in July we made all the final assembly and launched the boat. The boat ran OK from the ramp to his house. It has been in the water since then with little use because it develops miss after cruising for a while. Are thoughts are it was an issue with the carb.

This gets me to today, even though it was cold and raining today we went to his house to help him with some wiring and swap carbs to the one from his old engine. Back when we launched the boat we set timing, other than that I do not know much about the state tune of the engine( plugs Cap rotor??). So we swamped the carb to another Rochester 4 barrel. She fired up and idled nice and smooth, so we decided to venture out in the rain to see how she ran. We idled out of his canal through the no wake area with no troubles. Once clear of no wake we throttled up onto plane and were cruising around 3500 RPM at 22 MPH after a few min we noticed a slight drop in RPM, them it would pick back up. He then pushed the throttle to just above 4000 RPM and she sounded like it was pinging (detonation) so I had him back off to around 3500 RPMs where it ran good again for another few min. Then like before the RPMs would drop then come back up but this time is started popping from the carb, but now more throttle caused it to bogged. We also had a bad exhaust odor in the cockpit (full camper top up). Back at no wake speed she ran fine. BTW it seem better with this carb than the one that came on the engine. Back in July when we were out and the missed developed we opened the fuel fill to make sure the vent was not clogged. The screw on fuel filter is new and the one we checked the one in the carb before installing it. He is buying a rebuild kit for the Carb so I will go through it when it comes in. What else could cause this? Ignition? Of course we sold his old one. Sorry for the long winded post but I am trying to provide the most info I can to help trouble shoot this problem. All this because he changed the name without the proper ceremony, LOL. As always any advise is appreciated. Have a great night.

 

[Baylinerchuck]

Just a few clarifications? Did you set the base time 8 degrees btdc? Did you ensure the TB IV was advancing per the chart when throttling up? Where did the replacement quadrajet come from?

 

[mr300z87]

Yes base timing at 8 degrees per Merc Maunal. I did not have timing light with me today to check advance will bring it next time. The replacement carb came off his old engine that was running fine before it ingested water from what I think wsa a blown head gasket. The carb that came on the engine is an unkown, I do not know the history of this engine. The one we put on today was working fine on the old engine but had been sitting in my barn for the last year. Do the TB4 ignitions fail when they get hot? Water in the Gas? We should remove and dump out the filter to check. It back to work tomorrow so no boat fun. But may be splash 2 on my project.

 

[alldodge]

Agree remove and check filter contents. Need to find out if the carb is getting enough fuel and if it is flooding, I'm guessing its running out of fuel at higher rpm. Need to check fuel pressure with it running, should be 4-7 psi. Have you check the anti-siphon valve?

 

[mr300z87]

Thanks for the response AD, what is the best way to check fuel pressure as after the rubber line from the tank it is all metal lines with compression fittings?. Also how do you check the anti-siphon valve? My understanding is it is a 1 way check valve threaded into the tank output port. He did order a rebuild kit for the carb so will probably do that before we try again and do some more trouble shooting.

 

[alldodge]

The gauge has to be between the fuel pump and carb. There is different ways, the easiest is pick up one of these but it needs a barbed fitting instead of the shrader valve as shown. It can be attached right at the carb.


Another way is remove the steel line and install a rubber line with a Tee fitting. Most the fittings you need can be found at an auto, marine and sometimes hardware store. Bring the carb fitting with you to check size. Another way is to get one fitting for the carb side and make a Tee using rubber lines and push the hose over the end of the fuel line going to the carb and use a clamp.

 

[mr300z87]

Any Idea what the thread size is on the carb? 3/8 flare? I know I used a 9/16 line wrench on the flare nut. I tried searching for something like in the photo you posted and found nothing.

 

[alldodge]

I think it is 3/8 flare, but here is the actual item and the Merc part number is 91-18078

 

[wellcraft-classic210]

Sounds much more like a fuel supply issue -- // Check valve leaving tank will catch Debris & clog / partially clogged Fuel filter(s) and or a line restriction are another possible root cause. // Low fuel pressure could cause the issue as well.


Keep in mind that a restriction will cause a pressure drop as flow increases. Checking the pressure under a heavy load will tell you. That's going to require some sort of tee fitting.

Debris in the float bowl could be the culprit if fuel pressure is good under heavy load but given that you have swapped the carb I am betting its fuel supply..

Good luck

 

[mkulin]

Thanks for the response AD, what is the best way to check fuel pressure as after the rubber line from the tank it is all metal lines with compression fittings?. Also how do you check the anti-siphon valve? My understanding is it is a 1 way check valve threaded into the tank output port. He did order a rebuild kit for the carb so will probably do that before we try again and do some more trouble shooting.

I have a 1981 mercruiser 260. The previous owner had similar problems. Removed the floor to expose the fuel tank removed the antisiphon valve (main gas line attaches to) and was full of dirt. IMHO on a boat that old I would suspect a dirty Fuel tank. We are running 4-5 mins bogging then cleaning the siphon tube and valve. The alternative is to have the tank replaced or cleaned both in the 6-900 dollar range so we just suck it up and clean it out. Ive got the procedure down to about 2 mins. Will Idle and low RPM all day but soon as we started sucking some gas at higher RPMS the tube would catch all sorts of debris.

 

[mr300z87]

Thanks for the responses. Those are all things we need to check. The funny thing is that we did not have this issue with the rebuilt engine. Before it ingested water is ran great. I couple things we can try next time out are shutting the engine off for a minute to let debris settle and see if temporarily solves the issue and then try running it on a outboard type tank to remove the boats tank from the equation. I know he is not going to want to replace the tank as he is looking to get her running and sell it. Removing the tank on one of these older Sundancers requires the engine comes out first to make room for the tank to slide forward to be removed.

 

[FreeBeeTony]

I would try running it on a separate fuel supply (maybe an OB tank) to eliminate anything in the tank or fuel supply line.

 

[NHGuy]

Some boat engines run that way when there is water getting in the combustion chambers. How did the previous engine get it's damage?
If you saved the exhaust manifolds from the damaged engine you had better have changed the gaskets between the manifolds and the risers. If any water leaks in there you can ruin the good engine. To check inspect the spark plugs. If any are super clean they are getting water-cleaned, which is bad. Go over the exhaust manifold checks that can be found in the stickies at the top of the forum.
If they are all normal looking be happy and continue checking as if it's a fuel delivery problem.

 

[mr300z87]

If I am not mistaken the manifolds and elbows came with the engine in the boat now and what came off the dead engine is in my barn. We inspected and replaced the gaskets with (Mercruiser parts) during installation. My engine injested some water this past summer and met it's fate, but it acted very different then what we have going on here. I will look at the plugs next time we go work on it anyway. Not really thinking about his Scow, as he calls is, right now tomorrow is splash 2 for my project.

 

[NHGuy]

Then I am probably off base. But hmm, what would make it run badly like it does?

 

[mr300z87]

I have not been back to my friends boat in a couple of weeks but yesterday I took apart the carb that originally came with the new engine when he bought it and will be rebuilding it. All in all the carb seemed in decent condition, but the one thing that stood out that was a lean causing issue was the secondary metering rod were stuck and require some force to remove them. This is a big heavy boat and requires every bit of the 260 hp to get it up on plane and to keep it there. So if the secondarys open but no additional fuel is added because of stuck metering rods would this not cause condition? I should have the carb back together this weekend but not sure if I can get to his place to install and test. Plus the weather calls for cool and windy here in NJ this weekend.

 

[Baylinerchuck]

Since the secondary jets and air valve are mechanically connected I think the jets would keep the top flap from opening. Certainly the jets being stuck would not allow enough vacuum to open the air valve. So the carb would not get much additional air through the secondaries. I would guess it would act like a two bbl carb in my opinion.

 

[boatman37]

even less than a 2 bbl i believe. a 2bbl should have bigger bores than the primaries on a Q-Jet?

 

[NHGuy]

Right, the secondary rods raise as the air doors open. And conversely the flaps won't open if the rods are sticking in the jets. Once they are free the flaps will be opened by vacuum, then initial secondary fuel will get pulled in from the secondary reservoirs, and by the time the engine increases speed the raised fuel rods will allow an undisturbed flow of fuel to the secondaries.
There's a good chance this will end your detonation & ping.

 

[jimmbo]

The only times I have ever heard of the Secondary Metering Rods sticking in the metering orifices are from serious gum/varnish/dirt, or they were badly bent. I've seen many air valves stuck from the forementioned items too. The rods can be removed by removing a small screw on the hanger between the two air valves. Bad gasket between the bowl and top of carb can cause havoc as can loose or missing emulsion tubes.