Rochester Carb Question

[JustMrWill]

I have been working on resolving a bog down at 3/4 throttle problem since I got this boat. We rebuilt the carb over the winter and it has gotten MUCH better but I am having some trouble getting idle mixture screws right...so I decided ot recheck float hight and all the other specs on linkages/choke etc. Everythign is in spec..BUT I noticed that the model number on my carb is NOT the one for my motor (4.3l)..it corresponds to a 5.0/5.7 liter engine.<br /><br />My questions:<br />1. What are the differences between the two models? Is it jets?<br /><br />2. Can the bogging problem be caused by the "wrong" carb?<br /><br />3. Is there anything I can change or should I be looking at a new/rebuilt carb?<br /><br />Thanks in advance.<br /><br /><br />-JustMrWill

 

[rbezdon]

Re: Rochester Carb Question

I have the exact same condition with the same carb but it is on a 5,7. I have replaced every fuel system including the carb with a new part and replaced every ignition part and reset (tuned) every thing to spec. Still there. I am about to pull the drive to see of there is anything in the exhaust (like a missing flapper).<br /><br />to more direcly address your question, from what I have found the carbs are pretty much identical. There might be a slight jet change but changing the jets does not seem to cure the bogging. I guess this makes sense as the bogging is a gross performance issue, the engine really slows, nearly stalls. The jetting only makes minor fuel mix changes, not fuel flow/delivery changes. My guess is look alse where like I am and save the money for a new carb. I spent it just to find it wans't related to the jetting setup. Few hundered bucks for nothing. Now I am following the advice of this Forum on many, many posts on this topic and checking else where.

 

[JustMrWill]

Re: Rochester Carb Question

Thanks for the response...I have been following your threads when I see them...I just replace the anti-siphon valve on the tank yesturday as well...we will see if things improve. I decided to go through carb and distributor 100%...checking dwell and timing tonight...carb is at 100% default settings right now...after dwell and timing are verified/set will go back to the tweak-n-test route with idle mixture. I just didn't want to be adjusting something that will never be right or that I needed to replace jets or something to get it right.<br /><br />-JustMrWill

 

[ron7000]

Re: Rochester Carb Question

<br /><br /> http://www.amazon.com/exec/obidos/t...002-5845285-5540800?v=glance&s=books&n=507846 <br /><br /><br />1) the difference is the cfm the carb is rated for. The v-6 only requires 300cfm at most. If you have a carb rated for a 5/5.7 v-8, that'll be at least 300cfm for a 2 barrel. I know the holley 4 barrel for the 5L 302 is 450cmf, and for the 5.7L is 600cfm. In either case, that's over/way over for a v-6 and would definitely cause fuel metering problems... most defintely at 3/4 throttle because that's where the main jets are doing the work and fuel is metered via vacuum through the carb. Carb cfm too big on small cfm motor = no vacuum = poor fuel metering = bog.<br /><br />2) YES<br /><br />3) verify the cfm of your current carb. If it's a 450/600 cfm carb, get a new one.<br /><br /><br />vatter41: get the book too. jetting is fuel delivery and theres more to it than just jetting.

 

[JustMrWill]

Re: Rochester Carb Question

Ok...thought I "saw the light"...ron7000 stated that my carb should be like 300cfm...sooo I started looking...and found this:<br /><br /> CARBURETOR HOLLEY 4 BBL-600 CFM <br /> Application OMC & Volvo Marine 4.3 Liter V-6 <br /><br />Why would they sell a 600 cfm carb for my engine if it wont work right? ARRGGGHH!<br /><br />So...how can I tell what jets I have in my current carb? And can anybody tell me what CFM I should be at? <br /><br />-JustMrWill

 

[rbezdon]

Re: Rochester Carb Question

Yes Ron 7000, those are the thoeretical max CFM those engines can pass through them based on the cubic inch displacement and the marine rated RPM. There is not a problem putting a carb capable of delivering more CFM if the engine could take it. In fact, OMC put the Rochester Quadrajet 750 cfm on the 4.3, 5.0 and 5.7 when none of these engines could consume 750 CFM. The 5.7, the largest of these has a max CFM consumption os 618!! The information you gave is some what confusing for someone with out a lot of experience. The OEM like OMC put different carbs on different engines and got the performance they advertise. No need to try to redisign what they did.

 

[rbezdon]

Re: Rochester Carb Question

Mr Will, dont get frustrated. The numbers thrown up by ron7000 are the max theoretical CFM for the engines at the max marine RPM range. There are tons of variable which make these not the best choices for selecting a carb. The OEM designed the engine for a specific performance and got it with the components they placed on the engine. Believe me, the Rochester 4BBL is standard OEM equipment for your engine. The actual carb you have may have been rebuilt or replaced on your engine with an equivalent to the original by a previous owner but there are many 4.3 running with the 4BBL carb which perform just fine. In fact, I would guess there was a day you or the previous owner of your boat was happy with the way it performed. Look into your boats history and I am sure you will find this is the case. Too big of a carb does not mean it wont work, just that you'll never full excersize the carb so buying a big carb is not the best way to get more HP unless you do something to improve how many CFM of air your engine can process.

 

[JustMrWill]

Re: Rochester Carb Question

Alrighty then....<br /><br />So...<br /><br />I should continue to tweak and test and get it running the absolute best it can...if that is still not acceptable then I should look into determining what jets I have on it...if "high" (600 - 750 CFM) I should look into replacing with lower (450-500 CFM) jets?<br /><br />Just wondering what the difference between the model listed for the 4.3l and the one listed for the 5.0/5.7l model...was it just the jets or was there other differences?<br /><br />One more question, started looking around for replacement carbs...mine is a divorce choke model...most that I see are electric choke...is it hard to convert my setup to use an elcetric choke model? I am just looking at options right now...prob not do anything drastic until next year.<br /><br />-JustMrWill

 

[rbezdon]

Re: Rochester Carb Question

Just to clarify, the CFM rating is based on the carb (housing bore mostly). The jets only control the amount of gasoline the carb mixes with the air passing through the carb. There is no way to adjust the CFM of a carb except to put a different carb of a different rating on. The just will adjust the richness (or leaness) of the air fuel mixture effecting engine performance. So changing the jets will not address the issues brought up by ron7000. Basically, you have to "redesign" the intake section of your engine by reselecting a carb and making sure you have the proper intake manifold etc. I dont recommend this for the beginner. Just assume the OEM designed it correctly and get it working as it should (and did when it was new). This should be possible. If you want to optimize from there, so be it. I did and I am sure there are quite a few others on this site wanting a few more Ponys from what they have. Then you also have the issue of adapting a new choke scheme as you bring up. While setting up an electric choke is not an extremely difficult thing, why chage?? Your current issues are not related to choke or starting and now you have to start working on a new issue and you haven't fixed the old. Again, your engine is mostly stock as delivered from the factory. It should be able to run OK as is. Once you have this, go from there. An electric choke is in some ways an advantage but it is not going to address your problem. Make sure all of the other things related to bogging are addressed. The antisiphon, venting, sererator, fuel pump pressure readings, exhaust restrictions. Tons of stuff. Look up the threads from FreebeTony and some of Don S's comments. He feels strongly about identifying the problem before throwing money and parts at it. I am guilty of the same and now I am finally going to pul off the drive and check the exhaust like he suggested to Tony.