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Carburation  Rating:  Rating
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 Posted: 06-25-2006 07:14 pm
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Bob 13902
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After rebuilding (Delta kits) both Stromberg carbs, and lots of follow-up troubleshooting, I can't seem to solve running rich at idle. Everything was rebuilt except replacement of needles and only 1 shaft seal was replaced on one carb. The engine is smooth and strong acceleration at above idle speeds, all the way to 7000 RPM. Sometimes at steady speed or deceleration, I feel slight surges. Moments after returning to idle, it begins to smoke terribly and the spark plugs become very black, sometimes fouling if sit at idle for extended periods. When the car is cold it needs no choke to start and stay running.

After rebuild, I had gas leakage and disassembled and reset floats, which essentially stopped all leaks except after running extended periods, I notice some gas seepage from the front carb choke...rechecked and lowered float more without seeing a change.  I did find a loose rear carb float support and peened it tight, but that didn't change anything. Lifting the piston results in an increase in speed; adjusted mixture 1 turn toward lean with no effect (had adjusted about 3 turns in an earlier iteration, with no effect, and reset to baseline before the 1 turn adjustment). I haven't re-checked the temp compensators (I set with hot air gun during rebuild), because I didn't think they could cause the problem I'm seeing. I assumed if not changing the shaft seals was a problem, it would make the engine run lean.

Any suggestions? Although the mixture needles had no obvious visual problem, could that be it? Is it likely that the new inlet valves are faulty - looked good. Floats seem to move freely, so it doesn't appear they are sticking. If I can't get success, anybody have a good suggestion where I can get some replacement carbs without problems?

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 Posted: 06-26-2006 02:53 am
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Mark Rosenbaum
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IMHO the most likely possibilities are:
  • Fuel pressure at carburetors is too high (normal is 2.5 psi, upper limit is 3.0 psi, IIRC).
  • Incorrect reassembly of the start enrichment valve or 'choke'.
  • Vacuum valves sticking in the elevated position when you release the throttle.
  • Incorrect mixture needle in one or both carbs.
Once any problems are corrected, the float heights, mixtures, and carb balance will all need to be re-set.

The temp compensators lean out the mixture when the engine warms up, so, as you surmise, it's very unlikely they have anything to do with your problem.

While you are correct that worn throttle shaft seals (as well as worn throttle shafts) will result in a lean mixture, the seals are a primary wear item and they really should be replaced whenever a carb overhaul is performed.

For replacement carbs, IMHO the least expensive option is to find a spare set on eBay.  JH-specific Strombergs are easiest to deal with, of course, but those from many other cars can be made to work if the correct mixture needles and vacuum valve return springs are used.

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 Posted: 06-30-2006 02:16 pm
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Bob 13902
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I checked the fuel pressure and it is slightly over 4 pounds...so that appears to be at least part, hopefully all, of the problem. There doesn't seem to be a way to adjust the pump that I can tell, nor can I find any online info on the pump. The brand name is Auto Pump made in New Zealand. Any suggestions on do I pursue a pressure regulator or just replace the pump? Do you recommended a particular pump?

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 Posted: 06-30-2006 10:03 pm
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Mark Rosenbaum
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There are several ways to address the fuel pressure situation.
  • Use a restriction type fuel pressure regulator in (preferably) the engine bay.  The versions I'm familiar with, have an adjustment screw that allows setting the outlet pressure as desired.  Many owners have reported satisfaction with this approach.
  • Use a bypass type fuel pressure regulator in the trunk.  Some JHs were originally equipped with a tee in the fuel line that fed a restrictor orifice whose output went back into the fuel tank.  The size of the orifice would determine the maximum pressure at the carbs.
  • Use a fuel pump that by design produces the proper fuel pressure at the carbs.  The original SU type pumps did this, of course.  Their replacements may be a bit pricey but the recent ones seem fairly reliable.  Another alternative is the Purolator / Facet pump sold by Delta Motorsports and others.  This is far less expensive than the SU pump but is quite noisy in comparison and a proper installation requires fabricating a simple sheet metal mounting bracket of some sort.  The JH takes the low pressure version which according to my records is model FEP 42 SV.
FWIW, I've had a Facet pump in my car for five or six years now and it has been utterly reliable.  I did follow the manufacturer's recommendation to relocate the fuel filter to the inlet line.  Attached is a photo of the way I installed the Facet pump.  A second fuel filter is installed in the engine bay, just before the tee to the carbs.  The measured pressure at the outlet of this filter (the tee's inlet) is exactly the desired nominal value.

Attachment: facet pump & filter.jpg (Downloaded 51 times)

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 Posted: 07-01-2006 07:04 pm
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edward_davis
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I have to agree with Mark's assessment.  I had a similar problem that was a result of my old pump alternately having no pressure and too much pressure.  I bought one of the new Facet pumps, a black Posi-flo pump rated for the lowest pressure, 1.5-4 psi.  This pump is a lot quieter than the older brass-cube pumps; it only makes noise when it's priming, after I initially turn on the ignition.  You can get this sort of pump at most auto parts stores, and they're relatively inexpensive.  

The new pump made too much pressure anyway, so I installed an adjustible pressure regulator in the trunk.  I agree with Mike, it would have been better to install it in the engine compartment; I had to use my pressure gauge to fine-tune the pressure regulator so that the pressure at the carbs was the appropriate amount, 2.5 psi.  I actually had to set the regulator slightly higher than 2.5 for there to be enough pressure at the carbs - at 2.5 psi, the carbs were sucked dry by the engine. 

My fuel system also has the bypass that Mark describes, but it wasn't enough to keep the carbs from flooding.  Since I have an aftermarket aluminium tank, I'm not sure that it has the correct restrictor.  I put my pressure regulator after the bypass and everyting works fine.

Here's a picture of my setup on JHPPG:

http://jhppg.com/gallery/1974-No-18713/DCP_0096

Here's the website for the Facet pumps:

http://www.facet-purolator.com/mcl/pages/frame_src/tech_frmsrc.html

I don't have any association with those folks, of course; I just got their pump to work in my car.

Be sure to check your oil for gasoline contamination after having the flooding problem.

Last edited on 07-01-2006 07:05 pm by edward_davis

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 Posted: 08-01-2006 02:46 am
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Bob 13902
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Mark, Edward...thanks for the suggestions...Well, it took a bit, but problems solved. The fuel pump was putting out over 4 psi and the choke disc was 180 degrees off. I fixed the choke assembly and installed a Holley 12-804 (1-4 psi ) adjustable pressure regulator ($25 + $10 ship/handling from Summit Racing). Although it has two outlet ports for installation with twin carbs, I chose to install it with one port servicing both carbs (upstream strainer filter) so I could put a gage in the other port if needed. As luck would have it, the regulator was defective and leaked by after the first day of use. Fortunately I re-checked the pressure and found the problem before tossing the carbs in the woods. The regulator was replaced, no hassle, and since I was a 1st time customer, they gave me a $15 credit toward a next purchase which I payed an extra $1.50 and got a gage.  Fortunately, this one worked well...and everything is chrome so looks good too. When fixing the choke, I removed the carbs to check the floats. I maintained the fuel in the bowl until disassembly so I could check the actual level in the bowls. The forward carb was controlling level to just above the "shelf" in the bowl. The aft carb level was much higher (1/4"+). I noticed that although the highest point was set at 0.688", the floats are not real consistent in angle and twist. I adjusted the aft carb which was the most "squirrely" to be more consistent looking as the forward carb but still 0.688" at high point. This resulted in making it control the bowl level lower than it was. One question here is: does anybody know the "nominal" level (height) the fuel should be controlled to...couldn't find that in any Stromberg books. Regardless, things are running well and smoke-free (as much as can be with a Jensen-Healey)!

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 Posted: 08-01-2006 04:16 am
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Mark Rosenbaum
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Congratulations on getting your car sorted out.  Problems caused by multiple malfunctions are always frustrating.

Your observation that the relationship between fuel level and float height setting is not necessarily consistent, due to variations in the floats, was not something I'd previously given any thought to.  Obviously it's significant, and I've learned something new here, for which you have my thanks.

Unfortunately, the only mention of  the correct fuel level in the float bowl that I've ever seen was a sketch showing it to be not quite to the top of the bowl, but without any dimensions attached.  I'd guess that the level should actuall be slightly less than that -- perhaps halfway between the floor of the shelf and the open top of the float bowl, but I don't know for sure.  Perhaps someone else has can add to this?

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