Hesitation

[jhorton3]

Any time I stomp the gas pedal the engine has a severe hesitation in throttle response. The engine stumbles, looses a couple hundred RPM. If I keep my foot on the gas the revs eventually climb after 20 seconds and it smooths out a little around 3,000 RPM. This happens if the car is standing still or moving.

Distributor (25d) rebuilt unit from local parts store. New cap, rotor, points, condenser, and coil. I've swapped condensers and coil with same results. This has been a problem for quite some time. Last weekend I put the timing back to static timing. Adjusting the timing helps slightly, but not enough.

I've got 2 SU HS2 carbs that were rebuilt units I purchased. The jet adjusting nut is down about 12 flats. This happens with or without air cleaners. I put 20/50 motor oil in the dashpots. I've got a clear glass fuel filter showing clear supply of fuel. This also has a square body fuel pump (Facet) that runs all the time no matter what.

Plugs and wires are Autolite brand. Plugs are gapped .25. Points are gapped .016.

So what have I overlooked? I'm planning on trying to solve this problem this weekend. Could it be carbs are too rich/lean? Another timing issue? I've considered getting a Pertronix Ignitor unit, but not sure it would really solve anything.

 

[jhorton3]

I found most of the problem. Turns out the carbs were way too lean. I cranked the jet adjusting nut down about 5 more flats. Idle is still a little rough, but when I stomp the gas it revs freely past 5,000 RPM without blowing any black smoke out the tailpipe. I think that's a good sign.

Also readjusted the timing. I had it a bit too far advanced, and discovered one of the bolts holding the distributor down was loose.

Just took her out in the neighborhood for two laps. She did the best she's ever done.

I've discovered another oil leak in a very inconvenient place. Oil is now dripping out of my oil pressure gauge and onto my left leg. Good thing there's no carpet in yet. Not sure if it's around the pipe connection or inside the gauge itself.

 

[piman]

Hello Jim,

the sort of hesitation you describe is typical of a weak mixture, although a retarded timing will sometimes give a similar result, but I'm sure your action is correct. I'm also sure that you are pleased that you didn't try and 'fix' it with electronic ignition.
With S.U.s, the 12 flats is generally around where you should be, but that depends on two factors:- the needles being fitted correctly in the pistons with the shoulder of the needle flush with the piston base and when screwing up the jet to the full up, you need to remove the dashpot\piston assembly to visually set it flush with the bridge, not just screw up the adjuster until it goes tight.

Alec

 

[jhorton3]

I've been lifting the piston up and cranking up the jet until it's flush with the bridge. Then I cranked down. I don't think what I considered flats was a full flat. The adjustments so far did improve it considerably last night. I haven't put the round foam air filters back on, so I might have some more adjustments to make. We'll see.

 

[jlaird]

The 12 flat rule is just a starting point for new rebuilds, it will without question be too rich. Do a real adjustment and get em right before you put the air cleaners on.

 

[piman]

Hello Jack,

"it will without question be too rich".

I'm sorry Jack, but that is incorrect, it's nothing to do with being a 'starting point' If there is serious deviation from that setting, something else is wrong. Also you need to have the air cleaners on while checking the mixture to have the carburettors operating as they will be run.

Alec

 

[jlaird]

Hmm, not on mine, shrug.

 

[middleagecrazy]

Twelve flats or Ten flats every one is different. There are many diffrent things that effect the adjustment. Compression, Cam, Valve adjustment,timing, Carb ware, fuel pressure and there are others. In my case the thing would not run at twelve flats. I started at eight and leaned it out from there. There are a number of issues that can case hesitation. One of which can be high float level that can cause one carb to dump more fuel than the other carb. Even when they are thought to be adjusted the same. Timing can cause the hesitation as well as the vacuuam advance. Heck even the plug wires and plugs can cause it if it has been running rich for a while. Now I'm not an expert but I have driven my 69 1275 Sprite just under sixty thousand miles in the last two and half years and I average 45 miles to the gallon running seventy to eighty on the interstate every day. Once the carbs are right they require adjustment very very rarely. I have not ajusted the jets on mine for over two years.

Anthony

 

[jlaird]

/bcforum/images/%%GRAEMLIN_URL%%/iagree.gif

 

[jhorton3]

45 miles to the gallon?! Wow! I think I'd find a hard top on Midgie and have her as a daily commute car if i could achieve that. Shoot...I'd be pleased as punch if she got 30. Though with the gas savings I'd be able to save up for a 5 speed conversion. Hmm....SMACK! Back to reality Jim.

 

[piman]

Hello Anthony,

thank you for explaining the many reasons that I grouped simply as 'something else is wrong'.
For a standard engine in specification the 12 flats will be very close. I agree that once set up, little maintenance or tuning is required with s.U. carburettors. Bear in mind that these deceptively simple instruments are made very accurately.
I do disagree with the float level point, as long as it is not over high and flooding, the level does not affect mixture, fuel is sucked from them not, as you put it 'dumped'

As a parallel example, my current car is a fuel injected Triumph. The metering units for these engines are available reconditioned from specialists and they are set up for a particular model on the bench. They are not easily adjustable and the majority of people just fit them and off they go with a nicely running car. In other words, no tuning of the fuel system.

Alec

 

[kcbugeye1275]

I agree with the adjustment once you get it set. I'm blessed wih great mechanics in my group of friends and once things get set, they don't need fixin. You don't see me giving tech advice, but once set, leave it alone and fix the real problem.

 

[middleagecrazy]

Alec
You are correct and I agree with you. I may have caused some confusion. The original issue was hesitation. I have found that a malfunctioning needle/seat can cause a pressure issue where the fuel pump is accually blowing fuel through the jet to cause a flooding issue on only one carb/two cylinders that as a result causes a stumble and hesitation until the butterflys open enough to compensate. This can really be an issue for someone that has just started getting one on the road and are trying to adjust one that as you said has Something Else Wrong. This was something that I have experianced twice. Once on my Sprite 1275 and once on the Triumph Spitfire. In both instances without adjusting anything on the jets the needle and seats were replaced and the problem went away. I think that it is one of those things that if it was running well and then the problem started it is more than likly not an adjustment it is usually something broke or just quit working.

Anthony

 

[middleagecrazy]

Twelve flats or Ten flats every one is different. There are many diffrent things that effect the adjustment. Compression, Cam, Valve adjustment,timing, Carb ware, fuel pressure and there are others. In my case the thing would not run at twelve flats. I started at eight and leaned it out from there. There are a number of issues that can case hesitation. One of which can be high float level that can cause one carb to dump more fuel than the other carb. Even when they are thought to be adjusted the same. Timing can cause the hesitation as well as the vacuuam advance. Heck even the plug wires and plugs can cause it if it has been running rich for a while. Now I'm not an expert but I have driven my 69 1275 Sprite just under sixty thousand miles in the last two and half years and I average 45 miles to the gallon running seventy to eighty on the interstate every day. Once the carbs are right they require adjustment very very rarely. I have not ajusted the jets on mine for over two years.

Anthony