Rochester 4 bbl, hesitating at WOT

[Evinrude Boater]

1987 4.3 Merc with Rochester 4 barrel carb rebuilt last year. At WOT and during holeshot it hesitates a little. I removed the spark arrestor to observe the operation and the accelerator pump is working, the secondaries are opening and fuel is flowing through each bore but it seemed to have a lean pop. How far are the secondary flaps supposed to open, vertical? Mine don't, only about 45 degrees. Yes, I checked the fuel filter and it was clean with no sign of water.

 

[Alpheus]

Re: Rochester 4 bbl, hesitating at WOT

How about the fuel pressure???

 

[Evinrude Boater]

Re: Rochester 4 bbl, hesitating at WOT

How about the fuel pressure???

I don't know, I don't have the tools to check. What do I need to use once and throw in the bottom of the tool cabinet? From observing the fuel spraying down the throats it seemed to be getting loads of fuel.

 

[NHGuy]

Re: Rochester 4 bbl, hesitating at WOT

believe you need 5 to 7 PSI

 

[Joshua Nichols]

Re: Rochester 4 bbl, hesitating at WOT

45 Degrees?? That doesn't seem right..

 

[sea flower]

Re: Rochester 4 bbl, hesitating at WOT

Just went through this myself. My carb was destroyed by the eythenol. EYTHENOL sucks for boats. Any little piece of grap can clog a jet and cause these problems. I replaced the carburetor with a reman one and it didn't work either. I couldn't find any way of making adjustments on it so I found a cheap one on ebay and rebuilt it myself. You may have to open it up and clean it again. I was chasing the same problem for over a month. filters, strainers, new gas, but a mechanical fuel pump works or doesn't. There's no maybe. So that wasn't my problem. I just put the rebuilt one on Sunday and it ran like it suppose to. I have an 83 Sea Ray Express Cruiser 260 mercruiser.
Carburetor!!!!

 

[Aloysius]

Re: Rochester 4 bbl, hesitating at WOT

The air valve may be opening too early.

Ethanol tainted fuel will cause a carbureted engine to run lean, as the ethanol is added strictly as an oxygenator to replace MTBE.

a 4.3l engine probably isn't big enough to open the air valves on a quadrajet fully, since their function is air flow controlled.

 

[Evinrude Boater]

Re: Rochester 4 bbl, hesitating at WOT

45 Degrees?? That doesn't seem right..

The photo shows the flaps in the WOT position. I don't knowingly buy ethanol blended fuels for my boat.

 

[Joshua Nichols]

Re: Rochester 4 bbl, hesitating at WOT

Ok I thought you meant the secondaries wasn't opening more that 45 degrees.. So I think you should be okay if they are.. What you are showing are not the butterflies for the secondaries. All quadrabogs have those on them and I think that's as far as they open..

 

[Joshua Nichols]

Re: Rochester 4 bbl, hesitating at WOT

Sometimes the timing being off a hair will cause a little pop.. A vacuum gauge will tell you alot about a sick engine

 

[zbnutcase]

Re: Rochester 4 bbl, hesitating at WOT

How far the secondary air valve opens is determined by the app it's calibrated for. It doesn't open very far on the 3.7 4BBL, either.

 

[bigdan1]

Re: Rochester 4 bbl, hesitating at WOT

being a quadrajet mechanic ...
1) with engine not running , cable undone to carb ,
manually push carb to wot !

a) does the lower buterflies open fully ( look true upper ones )?
b) manually open the upper buterfly to its limit : do they open more then 45 deg ?

if both butterflies open to 90 deg manually , then the 45 deg mentionned before hand is normal , those are activated by engine demand


if the upper butterflies open manually to 45 deg only , look under the side of carb there is a tang that limits the butterfly travel : ajust acordingly

be sure also that the boat throttle cable travel goes to the point of openeing the lower buterflies fully open ...ajust acordingly if not

 

[PitchFork]

Re: Rochester 4 bbl, hesitating at WOT

Maybe the spring tension is too high? Mine was at one time and the butterflies would not open full and I had popping at WOT. So at WOT I pushed the butterflies to full open and the popping went away and RPMs went up to specs. Lowered tension on spring which is on the starboard side of carb.

 

[Alpheus]

Re: Rochester 4 bbl, hesitating at WOT

You need to check your fuel pressure and rule it out. Just because you see fuel squirting into the carb from the accelerator pump just means that there is fuel in the bowl but that doesn't mean that your pump is able to keep up with the WOT demand. I have seen many mechanical pumps that work but do not supply adequate pressure...

 

[Evinrude Boater]

Re: Rochester 4 bbl, hesitating at WOT

Sometimes the timing being off a hair will cause a little pop.. A vacuum gauge will tell you alot about a sick engine

I attached the vacuum gage on the line coming off the carb for the dash pot. The vacuum reading is no higher than 15 and dropping back to 14.5 when the engine hesitates or misses a little at an idle. The gage indicates late timing or manifold leak for that condition. Maybe this relates to my other thread about oil puddling on the intake manifold by tunnel #3. I have to get a timing light to make sure of the timing.
I won't be going anywhere until I get the gas line leak fixed.

 

[Aloysius]

Re: Rochester 4 bbl, hesitating at WOT

Missing at idle may simply be idle mixture related..15" on a marine cammed engine is pretty normal.

A recommended procedure.

1) Set timing. Preferred method is to set total timing using an advance timing light or timing tape, not simply setting initial and hoping it is correct.

2) Set idle speed

3) Adjust idle mixture for maximum manifold vacuum

 

[Evinrude Boater]

Re: Rochester 4 bbl, hesitating at WOT

A recommended procedure.

1) Set timing. Preferred method is to set total timing using an advance timing light or timing tape, not simply setting initial and hoping it is correct.

Can you point me to more information on this procedure? My Mercruiser manual doesn't describe this.

 

[Aloysius]

Re: Rochester 4 bbl, hesitating at WOT

The manual won't describe it. The intent is to get the desired advance at operating rpms..30-34 degrees on non-vortec heads. The ONLY way initial timing works is IF you know EXACTLY what the advance characteristics of YOUR distributor are. If you have 12 degrees advance in the distributor, which is 24 degrees crankshaft (crank is 1/2 of camshaft) then 8 degrees initial works ok. All this advance should be "in" by 3000 rpms.

A "timing tape" is marked 0-360 degrees, so you can tell exactly what the advance is at any given rpm.

a 4 cylinder fires every 180 degrees, an even v6 every 120 degrees, and a v8 every 90 degrees.

 

[Cpt D]

Re: Rochester 4 bbl, hesitating at WOT

Fix your fuel line leak first!!

 

[Evinrude Boater]

Re: Rochester 4 bbl, hesitating at WOT

Fix your fuel line leak first!!

The new fuel line is being fabricated so no work on her for a day or so. Stand by....