1988 4.3 OMC with Rochester Quadrajet carb issue

[lasko1]

The quadrajet carb on my 4.3 OMC was replaced ~4 yrs ago due to an air leak after a rebuild which wouldn't allow us to adjust the low idle mixture jets. The mechanic intentionally left the linkage off of the throttle side of the carb so the 4 barrels wouldn't kick in and said that would give me better gas mileage. I reluctantly tried it once on the water and immediately requested he put the linkage back on since I couldn't plane out very quickly and I lost 10 mph of top end speed (32mph with 2 barrels vs 42 mph with 4 barrels.

Ever since the carb replacement, I have a studder/bog issue when I go over 32 mph (4 barrels beginning to engage I believe). I could put it to WOT and get out of the hole fine, but running between 32-37 mph I get the studder. At WOT, the boat runs 4600 rpm smoothly.

I recently replaced points, condenser, rotor, small fuel filter at carb, and fuel/water separator due to severely poor running engine and everything is back to normal with the engine....and the studder from 32-37mph is still there. Points set to .017 gap to achieve 38 degrees dwell. Timing set to 6 degress BTDC @ 1000rpm.

The engine runs like a champ other than that small throttle range so I feel I am getting sufficient fuel delivery and have 6 psi coming from fuel pump. The only suspect thing I have found so far was the linkage rod on the starboard side of the carb that connects from a vacuum part to the secondary butterflies was a bit "sticky" when I manually opened the secondaries. I put a very small amount of grease in the groove that the rod slides through and that smoothed out the secondary operation.

I have not had the boat on the water since, but am looking for other possible causes until I can see if the sticky linkage was the issue.

Here is a pic of where the rod was sticking before I applied the grease.


Any suggestions or help is appreciated.

 

[Silvertip]

The secondary air valve shaft has a small spring to tension the air valve. Tension is adjustable. Adjusted wrong and a bog can develop if the secondaries open too early. Poor performance or delayed full throttle rpm if the tension is too high which delays opening.

 

[lasko1]

Update: Finally got the boat on the water after putting the grease on the secondary air valve shaft slot shown in the original post above with the red arrow. It "seemed" to lessen the studder when running above the 32 mph but the studder was still there. It was too rough to drive around with the flame arrestor off and engine compartment open in order to see what the secondaries were doing at different throttle positions, but I'll try that next time out.

Also, the engine did seem to have a little less power when going WOT to get on plane. I'm talking very little difference but I would like to make sure everything is tuned properly.

I'm pretty sure my gap and dwell are set properly (.017 gap to achieve 38 degrees dwell), but is my timing of 6 degress BTDC @ 1000 rpm right? I'm going off the dash tach. What would the degree reading drop down to when in neutral with idle @ 600 rmp? Mine read 0 - 2 deg BTDC but will recheck everything soon.

Lastly, could the secondary metering rods be the wrong ones for my carb? If so, what would the performance act like? I'm thinking that since the boat runs 4600 rpm and smoothly at WOT, the metering rods would be fully raised and allow full fuel to flow. But, at 3/4 - 7/8 throttle, maybe the metering rods have the improper taper/size and could be causing the studder....just guessing here. Again, this is not the original carb and was replaced 4 yrs ago when the issue first showed up.

I will look up my carb serial # when I get home and see if I can find what secondary metering rods are supposed to be in there.

 

[bruceb58]

Is that a marine carb?

 

[lasko1]

Gosh I hope so. How do you tell? 4 years ago, the guy cleaned and rebuilt my original carb due to accelerator pump issues and the boat bogging when nailing the throttle to WOT. After the rebuild, there was an air leak on the starboard side of the carb and he couldn't get the low speed idle to adjust, so he went to his shop and showed up with the replacement one. It looked exactly like mine except for the 6 or 8 screws that hold the carb together. The heads of the screws on my old carb were slotted for a regular straight blade screwdriver....the heads on the current carb have allen screw heads. That's how I knew it was a different carb.

Will I be able to tell if its a marine carb by the serial numbers?

 

[bruceb58]

Is the clear tube that carries fuel from the fuel pump when it fails hooked up to it?

 

[jimmbo]

"It looked exactly like mine "

What it looks like on the outside and what is on the inside are two different stories. I wouldn't have accepted the swap, and would have demanded the so-called mechanic to fix the original carb. Either he used the wrong kit, doesn't know anything about carbs, or needed a carb, made yours look defective and gave you something else.

In your second post you're speaking about metering rods being fully raised. The primary ones yes, but you can't see them. The secondary ones are opened in relation to the secondary air valve opening. Your 4.3 will not open that air fully unless it can turn about 9 of 10 thousand rpm. The linkage your originally greased is hooked to the choke pull off and is designed to hold the airvalve closed. When manifold vacuum drops the choke pulloff is designed to slowly release the airvalve and reduce the chance of the air valve opening too soon or too quickly thus preventing a bog or hesitation. Stickiness of the airvalve is usually dirt and varnish on the shafts, a bit of carb cleaner usually frees it up
Your stumble, started after the replacement was bolted on? without the original carb, only a Deity would know the calibration of the carb. There has to be a number stamped on the fuel body just below the airvalve shaft. With it you might be able to get some info as to it's original use was.

Was this at a dealership? or some clown claiming to be a mechanic?


I wonder what he did to create an airleak on the starboard side that couldn't be fixed?

 

[lasko1]

Got home last night and took a picture of the serial # on the carb which I posted below.

The rubber hose from the fuel pump to the carb is hooked up and is free of any obstruction since I was able to blow thru it (just used my breath).

My ol eyes read the serial # as being 17082403 3543

Hopefully someone knows where to look that serial # up and I'm hoping its a marine carb.



Here's another pic of the carb and the rubber hose in the lower left of the picture is the one that goes to the fuel pump and I can breath thru it.



The mechanic who rebuilt my original carb is not from a dealership. (Kicking myself for not going somewhere like a dealership that has a warranty). Anyway, the fella is a retiree that all the engine guys refer to as the "carb guru" and lots of folks go to him for rebuilds. He has worked on carbs that came off things ranging from race cars to Army helicopters. He said he was a mechanic in the Army and he even got to meet the President of the USA years ago when he worked on the presidential helicopter. With so many folks having used this guy for rebuilds and recommending him, I felt confident with him doing the work since I didn't know how to do the rebuild myself.

As far as the swap out of the carbs, I believe he felt pressed for time and wanted to get me up and running. The mechanic and I live quite a distance apart so we were doing the troubleshooting and repairs at a half-way meeting point which was at my uncles shop and it was getting later in the day. The mechanic rebuilds the carbs in his house so he took the carb back there after he found the air leak which was on the starboard side of the carb. I thought he was going to repair the leak on mine when he went home (not knowing how long carb rebuilds take) and bring it right back, but instead of taking the time to rebuild or re-gasket mine, he just grabbed this one and brought it back and put it on.

When he got back with the carb, I noticed it wasn't my original one and questioned him on it. This raised a major "red-flag" in my mind, but with me being inexperienced with carbs and him being "the guru" and saying it was a matching quadrajet, he installed it, adjusted the 2 idle screws on the front of the carb, and it ran fine on the muff's and that was it.

Its my fault that I didn't push the issue for him to repair my original carb and it's also my fault for not going back to him when I noticed the studder at higher rpms. I could try to get back in touch with him to see if he still has my old carb, but that was 4 yrs ago and I didn't keep the photo that I took of the serial # on the original carb.

Chalk this one up to a bad learning experience.

One last question: When setting the timing, I set it to 6 degrees BTDC with the dash tach at 1000 rpm's. When back at idle 600 rpm's, the timing goes down to 0-2 degrees BTDC. Is this correct?

Thanks for all the responses so far.

 

[bruceb58]

Looks like a marine carb to me.

Go to post #9
http://forums.iboats.com/forum/engi...es/595369-how-to-tell-if-i-have-a-marine-carb

 

[lasko1]

I did a little internet surfing too and concur that it is a Marine carb. I also found out that the serial numbers 17082403 mean:

1708 = 1980's model carb
2 = 1982 (the actual year of the 1980's)
4 = some California emissions standard.....a 2 in this position of the serial# would indicate some Federal emission standard
0 = Chevy engine
3 = would fit manual transmission car (odd number is manual tranny and an even last number is auto trans vehicle) so they say

Here's the link if anyone else is curious as to what their serial# stands for: http://www.carburetion.com/quadnumber.htm

Now, with my engine running good except for the upper mid-range throttle position (32-37mph out of a max speed of 42mph), do you think the secondary metering rods could be the wrong size and I'm either running too lean or too rich when the secondary air valves begin to open? Boat runs great at WOT and that would be when the secondary metering rods should be pulled as high as possible out of the jets and letting full fuel flow. But at 3/4 throttle, when the secondaries are only needing to be pulled open slightly, the metering rods may be the wrong size and not letting enough..or too much..fuel flow causing the studdering issue.