Carb Problem

[misalmon]

A few weeks ago I bought a new carb to replace the POS that had been acting up. Boat is a 1977 tiara w/ mercruiser 898 (chevy 305) and alpha 1 outdrive. It turns out the original is a rochester 2 barrel and I bought a mercarb instead because i'm an idiot. I obviously thought I ordered the right one, but I did not. It looks exactly same and the boat runs but I've been having some problems recently but I'm not sure if this the cause as many other parts have been replaced and fixed.

This is the carb that I received in the picture. It has the vacuum tube. The Rochestor did not. It came with a small rubber cap on it, but I think this is what's causing the problem.

The problem is rough idle and then sometimes it backfires once warm, especially with a heavy load going into waves. The only thing I can think of is this vaccuum tube or a bad gasket that is causing an air leak. I've taken it into a reputable marina/mechanic and they fixed the exhaust bellows, they said the last place used the wrong adhesive. Can I just plug this vaccuum tube or do I need to go back to the rochestor?

 

[dan t.]

Re: Carb Problem

The tube is for a pcv system, you can plug it or add a pcv set up, my personal prefrence would be to add the pcv, better for the motor and the enviornment.

 

[misalmon]

Re: Carb Problem

Thanks for the response, I appreciate it. I will definitely do that if I ever figure out what's going on with the engine, for now probably just gonna plug it. I'm gonna put a new carb mount gasket today and then try to find a vacuum with some wd-40.

 

[90stingray]

Re: Carb Problem

Its nice to have a vacuum port so you can dial in the idle mixture. Hook a vacuum gauge to it to see what the motor is doing. Just a thought...

 

[misalmon]

Re: Carb Problem

Just took off the mounting gasket and it looks like its beat up pretty good. Unfortunately I'm having a hell of time finding the correct gasket. Went to napa and autozone and the only gasket they found had 2 mounting bolts, mine has 4. So I think i got the right one on the way, but it may not be until monday. I'm thinking I will just dump it off at a mechanics and be out another 2 weeks, but at least I'll know I didn't screw anything up.

Could the mercarb having only 1 idle screw instead of 2 like the rochester be the problem?

 

[90stingray]

Re: Carb Problem

I doubt it being a mercarb is the issue. Its a carb just like Rochester... they both should do the job. If you have vacuum leaks you for sure have issues. You could cut your own gasket out of gasket material from the parts store... that way you don't miss the holiday weekend!

 

[misalmon]

Re: Carb Problem

Not sure if this was smart but reinstalled carb with existing bad gasket just to hook it up to muffs and see what was going on. It fired up and it doesn't like to idle, I revved it up in nuetral for 30-40 seconds giving it higher rpms and eventually it will stay in idle. I checked some things and then tried it again. This time it wouldn't stay in idle no matter what, maybe cause its warm and the leak is bigger?

Theres a large hissing noise which I'm sure is the vacuum leak, I tried using wd-40 to isolate the problem but I'm not very experienced with this sort of thing. Could that gasket really cause that big of an vacuum leak? Is it a more serious problem?

Thanks for all your help, I'm just trying to get out salmon fishing and tired of waiting for the shop.

 

[90stingray]

Re: Carb Problem

The hissing is probably the carb itself and the engine... that's normal. As far as rough running... prolly just the mixture screw needs adjusted. Make sure its about 1 and 1/2 turns out from seated. This will get you in the ballpark. If you hook a vacuum gauge to that port that's capped you can dial in the mixture perfect.

 

[John_S]

Re: Carb Problem

What engine was this Mercarb for? If it wasn't for an older 305, then jetting and adjustments could be off. I suspect that it is for a newer vortec, which has pvc system.

Did you have idling problems with the rochester?

 

[misalmon]

Re: Carb Problem

Here is a link to the purchase. http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=280664728660&ssPageName=ADME:X:RTQ:MOTORS:1123

 

[SSTKO81]

Re: Carb Problem

If you have any issues tuning it, theres a great article in the "Adult Only" section. I used it just today. I thought I was running fine, turns out I was way off. Good luck.

 

[misalmon]

Re: Carb Problem

The carb has been a problem for as long as I could remember, which is why I decided to buy a new one. It had similar problems of a rough idle and would sometimes act up not letting you get up on plane, sometimes it worked just fine. I went over and took a look at the mixture screw. I saw how many turns it took to seat and it was roughly 3 and 1/2 turns. I have a feeling this is the culprit, and its a game of getting it set right. I also took off the rubber cap on the pcv valve connection and when I do that it won't idle at all.

I guess tomorrow I'll haul it over to the lake with some tools and see if I can get it adjusted properly. I think I also need to make sure the cap on the vacuum tube is sealed tight, any ideas? Is a vacuum gauge the best way to tune this? If so how would I go about doing that?

 

[John_S]

Re: Carb Problem

The carb has been a problem for as long as I could remember, which is why I decided to buy a new one. It had similar problems of a rough idle and would sometimes act up not letting you get up on plane, sometimes it worked just fine. I went over and took a look at the mixture screw. I saw how many turns it took to seat and it was roughly 3 and 1/2 turns. I have a feeling this is the culprit, and its a game of getting it set right. I also took off the rubber cap on the pcv valve connection and when I do that it won't idle at all.

I guess tomorrow I'll haul it over to the lake with some tools and see if I can get it adjusted properly. I think I also need to make sure the cap on the vacuum tube is sealed tight, any ideas? Is a vacuum gauge the best way to tune this? If so how would I go about doing that?

The default starting point according to the SM, is 1 1/4 turns out. On any new carb, you need to do idle mixture, idle speed and choke adjustments. Keep that vacuum port sealed or a vacuum gauge on it. It will be way too lean with that open. The merc procedure doesn't use a vacuum gauge, but I always hook one up to my engine while doing any tune-up/adjustments. You can do a more precise mixture adjustment, but the bigger value is in additional diagnostics on your engine.

I'm a bit skitish of any ebay rebuilder. I had a bad experience with one. Won't try that again. Anyway, what is suspect is that they claim it is for both 5.0L and 5.7L. Take a look at a SM and you will see the 5.0L Mercarb is 35mm and 5.7L is 43mm. Power valves are different, too, but most of the other specs were in common. Which did they sellect and how well will it work? Don't know, but there shouldn't be a big imapct on idle. Since they are using merc cores, there should still be a merc part # that would tell you it is a 35 or 43mm. The internals would be up to them.

If you can't get it to smooth out and idle, look down the throat and see if you have any gas dribbling or pooling.

 

[misalmon]

Re: Carb Problem

Ok I was re-reading my post and forgot to mention something. Last time i took it out after getting back from the shop, when it does backfire, I ease off the the throttle and get a loss of power to the point it will cut itself off and die. It will restart just fine and I can rev it up in nuetral, but as soon as I move to idle to try to put it in gear it cuts right back out. Once the engine cools down it will run again with symptoms mentioned. Not sure if that changes anything but thought it was worth mentioning.

Thanks for looking that up and now I'm scared that the carb is just a POS. I'm still gonna take it to a local lake and see if adjusting it helps. I'll make sure I start with 1 and 1/4 turns out. Found the service manual from the adult section with the proper instructions for tuning the idle speed/idle mixture.

 

[John_S]

Re: Carb Problem

I suspect that you may be dealing with more than a carb or carb adjustment problem. Thats where the vacuum gauge may provide some additional info. Can I assume you have done a recent tune-up, plugs, dist cap, rotor, timing? Good compression readings?

 

[misalmon]

Re: Carb Problem

I suspect that you may be dealing with more than a carb or carb adjustment problem. Thats where the vacuum gauge may provide some additional info. Can I assume you have done a recent tune-up, plugs, dist cap, rotor, timing? Good compression readings?

Have done a recent tune-up, pretty much everything on there is new. New plugs, wires, dist. rotor, ign coil, bellows, etc. Have not gotten compression check. Took it out this morning and started adjusting. I got it to smooth out after screwing with the mixture and speed adjustments. I ran it in forward idle for 30 minutes and got it up on plane for another 30. Then I went on and off plane multiple times. I could not get it to backfire or cut out after 90+ minutes. I also did a better job of plugging/caping the pcv vacuum tube. So for now I guess it's fine, I will be going out fishing tonight and it will probably run like crap lol.