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Tuning a Carter carburator


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18 replies to this topic

#1 manoletr01

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Posted November 29, 2010 at 08:42 pm

Hi guys,

We finally replaced the sparkplugs on our tug. Unfortunately, it seems to be running very rich. The plugs get a coat of black soot even after just one minute of idle.

I discussed the situation with our mechanic and he said that he needs to open the top of the carb to adjust the jet. I find this a bit odd since this will necessitate repeated opening and closing of the carb in order to finetune the air:fuel mixture.

I looked at the carb and there seems to be two adjustments. An screw near the base that, I believe adjusts the amount of air flowing into the carb, and an idle adjustment screw on the other side. Shouldn't introducing more air into the system via the screw be sufficient to lean out the mixture?

I'm hesitate to let him open the carb until I'm sure he knows what he's doing.

Thanks!


Manolet

#2 68RT

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Posted November 29, 2010 at 09:19 pm

The only adjustment inside most Carter carbs is the float level adjustment. The one screw adjusts the idle speed air/fuel mixture and the other does the idle speed. It would be nice to know exactly what model carb you have as there are many more possibilities on the outside to adjust. That does not mean that you do not have other issues inside like the main jet having come loose. A cold engine will run rich because the choke will be on for a while unless it is a manual choke or you have overridden the spring applied choke. Also, unleaded fuel will give a more more sooty look than the older leaded fuel. I do agree that you probalby are too rich because a minute is too short of a time.

#3 ImperialCrown

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Posted November 30, 2010 at 04:16 am

On a warm engine, try adjusting the carb mixture screw in the throttle base plate in (clockwise) until the engine idle speed just begins to slow down and faulter slightly. This would be a lean idle. If the plugs are carbon fouled, sandblast them clean first, then blast with air to remove any sand particles.
The choke plays a big part in the first few moments of running and does enrich the mixture for cold engine operation. You just don't want it too rich. Make sure that the spring tension closing the cold choke isn't too strong and that the vacuum pull off opens the plate sufficiently and the vacuum pull-off diaphram or chamber isn't leaking or binding in any way. All choke linkage must move freely.
On a cold start, if you manually push the choke plate open further and the engine speeds up/smooths out, the choke is too rich.

Edited by ImperialCrown, November 30, 2010 at 04:19 am.


#4 dana44

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Posted December 1, 2010 at 12:09 am

It really sounds like the choke may be closed or partially closed to get the plugs that black that fast, or the timing is running retarded and needs advanced. Since the compression is probably in the 6.5:1 to 7.5:1 on the high end, gasoline octane shouldn't be a problem at all.

There should be a screw on the baseplate of the carb, located in the front area under the bowl. The second screw should be for idle as noted from previous posts. The one on the baseplate, if it is there, gently turn it closed until it stops, very gently. Then turn it out 1.5 turns. If this screw is more than two turns out, that's where the rich running is coming from.

The timing should be about 6-8 degrees advanced.

How does she drive, as in what happens when you drive forward, increase speed, back off the throttle and coast, things like thet? Black or blue smoke out the tailpipe?

#5 68RT

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Posted December 1, 2010 at 05:01 pm

Most of the flathead 6 engines were timed at a low idle with a pointer located in a hole at the rear of the engine that reveals a steel ball embedded into the flywheel. You matched them up and that is it.

#6 ImperialCrown

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Posted December 1, 2010 at 05:24 pm

Always check distributor point dwell angle before checking timing. For every one degree change in dwell, the timing will change one degree.

#7 manoletr01

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Posted December 2, 2010 at 11:04 pm

Hi guys,

Some developments. Apparently, the old mechanic didn't replace our carb despite us buying a new one. We replaced it this morning and our tug finally works. Yahoo! We still turned the mixture screw all the way clockwise then turned them back 1.5 turns as you suggested to make sure that we're on track.

Our only problem now is fine-tuning it. After letting it idle for 30 mins, we pulled the plugs and they're still covered in sooth, plus the exhaust is still black. I believe this indicates that we're still running a rich. We tried adjusting this by turning the mixture screw clockwise some more to lean it out but it doesn't seem to be affected. The idling stays the same regardless of how many turns we make, clockwise or counter-clockwise. You'll only notice a difference when the screw is totally removed from the base of the carb (turned fully counter-clockwise).

I looked at our choke mechanism and we seem to have a manual choke. It simply pulls the accelerator cable to bring the revs up. Since we didn't have any problem starting the tug, we kept the choke at a minimum for now.

Next stop is the timing. We'll advance it 6-8 degrees as recommended. We'll keep you posted on developments.

Again, thanks for the help guys!


Manolet

#8 68RT

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Posted December 2, 2010 at 11:43 pm

Here is the info on your YF Carter Carb. The 4371S was used on mid 60's Chevrolet 6 cyl. There are many pages of breakdowns. Your exact carb is not listed but the procedures are similar for all.

http://www.tocmp.com...s/p_ 33_jpg.htm

#9 ImperialCrown

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Posted December 3, 2010 at 07:01 am

With the idle mixture screw turned in, the engine should run leaner and rougher until it reaches the point of stalling. It shouldn't idle at all with the screw turned fully in.
If you have no change in idle, look down the throat of the carburetor for dripping gas from the venturi. If it is dripping, then this is the source of gas that is allowing the engine to keep running and not the idle circuit. This could also make it rich.
If you introduce a vacuum leak and the engine speeds up, it is rich. If anything, a vacuum leak should cause engine stumble and loss of RPM.
Gasoline saturated engine oil from prolonged rich operation might also keep the engine running rich. Change the oil if it smells of fuel.
The choke cable that actuates the accelerator linkage should also close the choke plate. If it is only raising the engine idle speed, then it is not a choke. Some of these sixes had an automatic (thermal bimetal spring) choke.
Start each test trial with cleaned spark plugs. They are 'self-cleaning' only if they can come up to temperature, run under a load (more heat) for awhile and have a proper air/fuel mixture. Carbon-fouled spark plugs may not give an accurate response to proper adjustments and will stay fouled at an idle.
After a good drive, recheck the spark plugs for carbon soot. Frequent starting and idle without a warm-up will tend to keep plugs fouled.

#10 dana44

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Posted December 3, 2010 at 11:11 am

I think the main jet is too large coming off a Chevy 235 or 250 cubic inch engine and needs to be replaced with a smaller one.

#11 68RT

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Posted December 3, 2010 at 11:51 am

I think the main jet is too large coming off a Chevy 235 or 250 cubic inch engine and needs to be replaced with a smaller one.


Main jet should not be moving any fuel at idle unless the float level is too high. You should be able to look down the top of the carb and see if anything is coming out the center venturi at idle (which should not happen at idle). From the pictures he sent me, I have noted that he has headers instead of a standard manifold. Therefore, there is no heat under the carb and until the engine is very warm, you need to be running a richer mixture than normal plus set the accelerator pump to its highest stroke or you will have a stumble.

RE: Imperialcrown comment. He may have a manual throttle cable besides a choke cable as this is not a standard application (airport tug)

I have now traced that #4371S to a 1967 Chevrolet (California spec)230, 250 or 292 (292 was in a truck only) w/manual transmission and it would have had an automatic choke when it left the factory I believe. I don't think that tehy had manual chokes available by that time.

#12 Dave

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Posted December 3, 2010 at 02:43 pm

That matches my lookup of the carb -- 1967 Chevy 235/250. Interesting choice to use headers. This may explain why the mechanic wants to open the carb though -- if the float level's totally off...

#13 Bob Lincoln

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Posted December 3, 2010 at 02:51 pm

Hi guys,

Some developments. Apparently, the old mechanic didn't replace our carb despite us buying a new one. We replaced it this morning and our tug finally works. Yahoo! We still turned the mixture screw all the way clockwise then turned them back 1.5 turns as you suggested to make sure that we're on track.

Our only problem now is fine-tuning it. After letting it idle for 30 mins, we pulled the plugs and they're still covered in sooth, plus the exhaust is still black. I believe this indicates that we're still running a rich. We tried adjusting this by turning the mixture screw clockwise some more to lean it out but it doesn't seem to be affected. The idling stays the same regardless of how many turns we make, clockwise or counter-clockwise. You'll only notice a difference when the screw is totally removed from the base of the carb (turned fully counter-clockwise).

I looked at our choke mechanism and we seem to have a manual choke. It simply pulls the accelerator cable to bring the revs up. Since we didn't have any problem starting the tug, we kept the choke at a minimum for now.

Next stop is the timing. We'll advance it 6-8 degrees as recommended. We'll keep you posted on developments.

Again, thanks for the help guys!


Manolet

If the mixture screw makes no difference in idle, something else is wrong that is overwhelming it. A manual choke should pull the choke plate open and shut. Is the choke fully open after a few minutes? If not, you will VERY shortly kill the engine by washing unburned gas down the cylinder walls, removing oil from them and ruining the rings, which will result in loss of compression. Make sure the choke is FULLY open when the engine is warm.

#14 68RT

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Posted December 3, 2010 at 07:08 pm

That matches my lookup of the carb -- 1967 Chevy 235/250. Interesting choice to use headers. This may explain why the mechanic wants to open the carb though -- if the float level's totally off...


The simple thing is for them to look down the throat at idle and see if any gas is coming out the main jet which empties into the center of the venturi. If the float is high, you will have dribble coming out the hole and that will definitely overwhelm the idle screw (low setting will not do anything but cause a lean condition). The rare but once in a while thing is the low speed jet air bleed hole is plugged and instead of just air being pulled through at idle, it will siphon fuel though that jet. The link below is for a YH which is the same carb but with the carb turned sideways. You can see all parts of the circuit" Low speed jet, Economizer, Air bleed, idle port, Adjusting screw, and the passages all laid out just like the YF but horizontally. The nice thing about a Carter is that you can actually follow the circuits start to finish easily.

http://www.oldcarman...YH_0011_jpg.htm

#15 dana44

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Posted December 4, 2010 at 12:30 am

Dribbling fuel would be a good indicator of the float too high, or the air bleeds being plugged, but the other diagnosis I mentioned, which is the ballpark method without a fuel/air sniffer to verify, is to determine if the engine is running too large a jet is with the idle screw. At 1.5 turns you are very close or right at optimum, plus or minus one quarter turn, whereas, as long as the needle itself is in good condition and is able to regulate the fuel bleed below the butterfly correctly, then all the way in without a change indicates the jet(s) is(are) too large, always checked at idle, so a smaller jet is in order if the air bleeds are not plugged and there are no vacuum leaks, which there probably isn't given the fact she is running rich, not lean, to begin with.

#16 manoletr01

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Posted December 7, 2010 at 08:33 pm

Thanks for all the feedback guys! This is really giving us a wealth of knowledge and a deeper understanding of our equipment.

I'll be discussing all these possibilities with our mechanic this afternoon and, hopefully, have everything sorted out by the end of the week.

Again, thank you so much for all the help guys. I'll keep everyone posted on developments.

I'll also send some pictures of our other airport equipment which some of you may find interesting. My friend's dad started the business back in the late 70's and they have managed to keep all their equipment in relatively good shape.

Hopefully, with your help, we'll be able to keep our trucks on the tarmac for more years to come.



Manolet

#17 Dave

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Posted December 12, 2010 at 05:29 pm

Aren't we due for an update? ;)

#18 manoletr01

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Posted December 12, 2010 at 09:09 pm

Aren't we due for an update? ;)


Hahahaha! You beat me to it Dave!

Our schedule last week was pretty tight so we only got to work on the least invasive (and easiest) tasks.

First the choke. We let the tug warm up and checked the choke plate. It seems to close properly and the vacuum pull off diaphram isn't leaking or binding.

We also checked the tip of the air:fuel mixture screw and it doesn't look damaged or blunted.

Our battleplan for the week is as follows:

1. Check if the oil smells of gasoline. If yes, change right away.
2. Check the timing at low idle. It should be 6-8 degrees advance. But before we can do this we need to find the pointer located in hole at the rear of the engine that reveals a steel ball embedded into the flywheel. These should match up. Sounds tricky but Im hoping it just sounds harder than it actually is.
3. Check distributor point dwell angle before checking the timing. Question though: What's the proper dwell angle for the tug?

If its still running rich at this point, we'll proceed with the following:

Venturi
1. Look down the throat of the carburator for dripping gas from the center venturi. This might mean:
2. The main jet might be too big. It should not be moving fuel at idle.
3. Float might be too high causing excess fuel to dribbling into the chamber.*

Low speed jet bleed hole
4. Check the low speed jet air bleed hole. It might be plugged and instead of air being pulled through at idle, it is siphoning fuel through the jet.*

* If it looks like we'll need to take the carb apart, we'll have to pause and order a Carter repair kit before proceeding.

I'd also like to thank the forum for the following documents:
- a cut-out diagram of a Carter YH carburator.
- a service procedure for a Carter YF carb.

Very, very helpful!

I'll keep you guys posted on developments. Again, thanks for all the help guys! Please let me know if I missed anything.


Manolet

Edited by manoletr01, December 12, 2010 at 09:12 pm.


#19 68RT

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Posted December 13, 2010 at 11:10 am

I believe all 230 cu.in. engines had a 39 degree dwell.


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