Moss Motors, Ltd

First let me describe the problem and then I will give the engine particulars. My problem is very “lumpy” performance below 2000 RPM. Acceleration and deceleration are fine but the car does not smoothly hold a constant speed. Above 2000 RPM the performance smoothes out but remains irregular. It acts like there are ”bumps” in either the engine torque or fuel delivery. In August Vimto (20524) described a similar problem. Motorbill66 suggested three possible causes, which I address below, but the thread never finished so I do not know if or how the problem was resolved.

 

My car is a rubber bumper MGB (stamped 1979) having LE accessories. I have reason to believe that the engine is not the original motor and it has certainly been rebuilt; i.e., an old parts list shows a Crane cam, compression is uniform but high, and the distributor is a 45DE4 usually found on ’78 or ’79 engines. I just replaced the 45DE4 with a new Pertronics Flame Thrower distributor. The carburetor is a manual choke 32/36 DGV which I have just rebuilt using kit parts. To avoid vacuum leaks I carefully torqued the carburetor down (with new gasket) to a rigid (not flexible) mount. I think that the two carburetor problems, discussed by motorbill66 have been eliminated. His third suggestion was “poor fuel delivery,” that he stated was “easily checked.” Can motorbill66 or someone help me with this measurement?

 

Other than the fuel delivery problem I have run out of things to try and would appreciate any new direction.

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BobK

My recommendation is to check your timing 7 ways from Sunday. But you have an engine far removed from standard, so "standard" settings are not likely to be spot on for you.

The Pertronix distributor, I've been told by a distributor guru (http://www.advanceddistributors.com/), has a general advance curve that may not be suited to your setup--vacuum advance pulled from the port on the carb rather than manifold being one possible issue (which does the Pertronix require?). How stable is the timing (i/e/ with a toiming light on the mark and running at various rpm, is the mark steady or does it jump around? Steady is good. jumping is not.

At an engine speed of about 3000 rpm your total advance (vac and mechanical) should be about 30-35 degrees. If the advance curve or vac advance is off then if set this way the car may have a hard time at idle.

 

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Old Fart
71 GT project
73 Roadster runner

Mac: Thanks for the response. Several of your comments on timing and the Pertronix distributor may be right on. The difficulty did begin with my changing the distributor but I have been wanting to blame something I did rather than the new distributor. I sent an email to Pertornix to get an advance curve but got no answer. I have measured the advance (with the vacuum line to the Weber disconnected) to 2000 RPM. I get 15 degrees advance above the idle (800 RPM) advance but the curve is not smooth. I have had trouble getting repeatable advance measurements with the vacuum line connected. I need to go back this weekend and make that measurement again; including, as you suggested, measurements of the timing stability. My memory of the measurement, several months ago, was that I was getting alot of jumping around as the RPM's increased. I am new at making this measurement and I need to learn to get repeatable values. At that point I would like to continue this thread.

You are right that the engine is "far removed from standard." However the performance was reasonable good before my attempt to "improve" things. Thanks for the help.

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BobK

Bob,

Check your fuel pressure first. It shouldn't be above about five pounds for that Weber. Now check the volume. I'm out of town and have no specifications in front of me, but I'd say that if the pump delivers at least a half a pint in thirty seconds or so, it's more than adequate for the purpose of diagnosis.

I'm trying to get a better sense of the symptom here, to help with the area to check. If you could describe in detail it would help. You know, accellerating? cruising? Jerking? surging? We'll get there.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Motorbill: Thanks for your response. I made the measurements you suggested this morning and sent a reply. However I do not see the post so let me try again. I am not too good at navigating this site.

 

I made the pressure and volume measurement at the output of the in-line fuel filter and with the engine off. I recently charged the battery. Both measurements were after the fuel pump was up to full pressure.

 

I pumped 1 ¾ pints in one minute. That is less than the spec; i.e., 18 US gal/hr ~ 2 ½ pints/minute. There was some sloppiness starting and stopping the flow but the error should be small over a minute. The pressure I measured was 7 ½ psi. The accuracy of this measurement is suspect since that was at the low end of the scale. A more accurate measurement will require a meter with a smaller full scale value.

 

Below 2000 RPM there is regular jerking in the speed. I need to gear down to maintain speed. Above 2000 RPM the performance smoothes out but some jerking persists. Enough to be annoying. I appreciate any thoughts or measurement you might think of. I am currently doing two things. First I am rebuilding my old distributor so I can compare it directly with the new Pertronix. Second, I am working at doing a better job of measuring the advance curve. The engine in the car has the timing marks below the engine and I want to try to transfer them somehow so I can make the measurement without crawling under the car each time. Thank again for the help

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BobK

Bob,

I've been out of "computer range" for a week or so. Now I'm back. If your fuel pressure really is 7 1/2 pounds, I think your problem might be right there...might. See if you can get a more accurate measurment. I think you have enough flow, even though it is not fully up to specification, to run without surging or jerking. If the pressure really is that high, it may be allowing carburetter the float valve to intermittently blow open and flood momentarily, causing this uneven running. Maybe yes;maybe no, but it's worth a thought. However, I still wonder, as have other posters, if you have ignition problems. Some of the earlier Pertronix units did some funny stuff.

Transferring the timing marks to the top is smart. We do it to every car we take in with the marks on the botttom. Since we use advance reading timing lights, we only have to mark TDC. It's simple. Using the bottom marks, set the engine to TDC. It doesn't matter which cylinder (1 or 4) is at TDC for this operation. Once it's there, we usually just clean off a spot on the pulley and a spot on the timing cover and place a small dab of white paint on each in a location where vision is unobstructed. Life gets just a bit easier then.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Bob-

High pressure plus low flow equals some sort of obstruction? Fuel line/pick-up/filter?

LCJUTILA 

LCJUTILA & Motorbill:  I should have known better than to report a measurement on the very edge of the meter. I found a reasonably priced fuel pressure meter with a 15 psi full scale value , rather than the 100 psi full scale I first used. Using this meter I get a repeatable 3 1/2 psi pressure. I also transferred the pulley TDC point so I no longer need to drive the front end up on a ramp and get under the car on every measurement. I am on the road for a week and then I will try to nail down the distributor advance & jitter values. Thanks to all who have responded to this thread.

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BobK

I followed Mac’s advice (“check timing 7 ways from Sunday”) to a solution. Actually it took two Sundays because the centrifugal advance of the Pertronix Flamethrower was so different from the 45DE4 that I replaced that I did not trust my measurements. The short version is that the Pertronix has much more centrifugal advance increase below 1000 RPM than the 45DE4. It is 15 degrees in 500 RPM rather than 8 to 9 degrees in 1000 RPM. Above 1000 RPM the two are "kind of" the same. When I changed the distributor I had dialed in the previous advance value at idle (650 at the coil). This resulted in way too much advance above 1000 RPM. On realizing this I dialed in the desired advance at 1500 RPM and now have my old car back. I do not know why too much advance caused the engine to act as it did. I thought too much advance would cause it to ping. In any case it is nice to be able to comfortable drive the car again.

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BobK

Bob

I just pulled the Weber DGV from my '80B [recently acquired] because, largely, of the condition that you describe.  I played with timing (20+ advance) and mixture with no significant relief.  I installed a pair of HIF SU's and am now happy.  I've since been told that float adjustment is critical with a DGV.  I'm sorry I didn't hear that before the swap but you may want to consider checking that.  Another issue that some feel critical is using the coolant heat fixture at the bottom of the (Cannon) intake manifold.  I didn't but I didn't find a temperature correlation.

Hi Barry:

Yes the float adjustment on the DGV requires care during a rebuild. The two floats available (brass and plastic) require different settings so you need to pay attention to which specification you are looking at. At one point there was a tool being sold to set the spacing. I just make up one when I do a rebuild and pay attention to getting the reference "wall" vertical. My cars performance has been enjoyable ever since I realized that the large centrifical advance change below 1000 RPM on the Pertronix Flamethrower needed to be compensated for. Actually the large initial ignition advance turns out to be an advantage with the DGV because it's primary venturi is all you have at start up. See www.mgexperience.net/article/3236dgev.html for a short description on this. Barry I do not recognize the "coolant heat fixture at the bottom of the intake manifold" that you wrote about. Can you refer me to something on that?

 

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BobK

Bob

 The Cannon intake manifod has fittings at the bottom to enable connection of the heater hose from the lower hose to the intake manifold and then on to the heater.  This is to raise the temp of the fuel charge which I've heard is a Weber issue.  I recently heard that it involves condensation of moisture out of the fuel charge in the intake manifold. [I'm only summarizing what I've read]  I have no personal opinion save wondering what is accomplished before the coolant heats up which was when I thought the Weber was intolerable.