Engine rebuilt high speed miss

[storm]

Have my '56 beetle stock engine rebuilt with new cam, piston and cylinders push rod-lifter, and rebuilt 28pic carb. Idles nice except a little ruff side. I installed engine and went for a drive 30 to 50 miles per hour. Noticed a miss once in a while. When accelerating flat spot then it caches and speeds on. Have 500 brake in miles so far. I did notice when going 45-50 the miss, I feathered the gas pedal and it improved some. Otherwise it pulls strong until it levels out. I wonder if the main jet needs to be richer? Or a condenser issue. Any ideals? George.
_________________
always ready to learn and give advice

[Eric&Barb]

Using just MPH ranges leaves out a lot of engine RPM needed info. EG @ 45-50 MPH, are you in third or fourth gear when you notice this problem??

Hesitation sounds like distributor problem with old gunky grease/dirt not letting the advance work as quickly as you need.

Carb has more than one jet. Each of the three jets has a different RPM function.

1. Pilot jet AKA idle jet, well works to keep the engine idling.

2. Main jet takes over from the idle jet at RPMs just above idle. If you do not notice a problem when going above the idle RPM, then both the main jet and accelerator pump system are working fine giving the engine enough fuel or possibly even too much.

3. Air correction jet which at higher RPMs allows air to be sucked into the fuel going to the main jet. This leans out the fuel mixture so as to prevent the engine from getting way too much fuel at high RPM. Unlike the pilot and main jets, the bigger the air correction jet the more it will lean out the fuel mixture.

So if you are in third gear at 45-50 MPH, which is pretty high RPM, then air correction jet might need to be a tad bit smaller.

Of course if by then you are in fourth gear it could not be the air correction jet at the lower RPMs of the engine. This is assuming you have stock gearing....

Other possibility is you have a loose/corroded connection going to the coil. Had kind of same missing in a 1961 beetle. Turned out the positive #15 terminal that was riveted to the coil had worked slightly loose causing loss of spark now and then and probably weak spark that did not ignite the fuel/air mixture in the cylinders as well as should have.
_________________
In Stereo, Where Available!

[Danwvw]

Yeah, that sounds like it's running lean. It could just need a fuel line cleaning or a fuel pump rebuild. Carb clean. etc.. before re-jet. Plugging the Air correction jet should clear it up it if so. Check float level it may be low.
_________________
1960 Beetle And 1679cc DP W-100 & Dual Zeniths!

[storm]

Turns out that the fuel in the carb bowl was dirty. Prices of what ever was on bottom of float bowl. Cleaned main jet, idle, and mixture screw jet. Went out for a drive and seems to have been cleared up. No miss,but a little hesitation. Will check vacuum at distributor. Thanks for replies. 45-50 was in 4th gear. G.
_________________
always ready to learn and give advice

[Eric&Barb]

Thanks for the update!
_________________
In Stereo, Where Available!

[storm]

Th '56 beetle is back. The carb is clean, fuel tank is cleaned and sealed, new fuel lines, filter. Starts fine when choke is on, but sounds like it is not running well, until it warms up. First thru third drives ok, but the little miss shows itself in fourth gear 40 to 55 mph. Not all the time but every so often. No popping thru the carb. Could it be a fuel pump that has a small tear in the diaphragm?. I do have another fuel pump, might change to rule that out. I've had this problem befor with another engine where the pump would not pump when the vehicle tried to go more than 50 mph. Gradually the speed decrease when the tear in the diaphragm got bigger. My current set up does not seem to operate like the other engine. if I get the same symptoms, then I will turn to the ignition parts. I liked to change one thing at a time so I know what made the difference. Any other ideals I should rule out? Thanks G.
_________________
always ready to learn and give advice

[Erik G]

at night in the dark, look for the cap or wires or something arching
_________________
Stop dead photo links! Post your photos to The Samba Gallery!

[Eric&Barb]

Could be a problem with the springs in the pump, or even the push rod is too tight when engine really heats up.
_________________
In Stereo, Where Available!

[Kjell Roar]

Does it reach top speed (72 miles)?
If the problem get worse at higher speed, check that the fuek tap deliver enough fuel. The rubber in it is known to swell and restrict fuel
_________________
I got a historic car, every scratch got a history...

My car, Aug. 4th, 1955 / an early 56: http://www.thesamba.com/vw/forum/album_page.php?pic_id=610438

[storm]

Update, solved. Danvw was right engine running lean. Looked at #1 spark plug looked very light brown to white. Took carb off looked at main jet 117.5. Swap out to a 122.5 measured with a jet gauge. What a improvement overall. Acceleration much improved and NO miss going 50-60mph. Thanks for all the responses. G.
_________________
always ready to learn and give advice