Cruising at 4,500 rpm...

[collins_dan]

...and the car is just not smooth. It almost feels like inconsistent fuel flow. I have fuel pressure regulator set on 2. I just replaced the dist cap, rotor and plug leads. I know I have a leak between the header and the block, but I can't imagine that would be causing the rough running. The car idles great, accelerates great, it's just when cruising at 4,500 rpm on the highway it hesitates, like its getting lumpy fuel. If I accelerate, it goes away. Which reminds me, I also recently added a fuel filter to correct this problem, but it has had no effect. The car is an S4 with Strombergs. Thanks. Dan

[paddy]

Have you checked the timing with a strobe? You might see some irregularity related to engine speed. Maybe worth a try, although fuel does sound the more likely area.

Paddy

[alaric]

Hi. Could it be a float chamber issue such as fuel frothing up or just simply the wrong float chamber level? With webers and dellortos the thackarey washers are supposed to isolate the carbs slightly. Is there something similar with the Strombergs?

Hope you get it sorted soon.

Sean.

[gjz30075]

Dan, how does the engine react if cruising at any other engine speed?

Greg Z
'72 Sprint

[collins_dan]

I need to experiment a bit more, but it seems to be very smooth accelerating at any rpms and very smooth cruising at lower rpms. Its only when I get to about 4,500 that it becomes uneven, or almost lumpy feeling. I need to get to some open highway and try cruising at various different rpms. I mostly drive around town and back roads, which are mostly accelerating and decelerating, which the car does great with. It seems to be only an issue when holding a constant speed at higher rpms. It will also hesitate if I try to accelerate through it, but then accelerate fine. Again more testing needed.

I was thinking that it was carb related, but that would seem like it would be uneven all the time. I seem to remember someone saying that if the o-ring had a slight leak, it could suck in more air at higher rpms, making the car run uneven, but I'm not sure who to check that.

Appreciate any ideas of things to test. Thanks, Dan

[ppnelan]

Check that both air pistons move up & down smoothly - if they stick at any point, this can cause a problem...

Matthew

[bengalcharlie]

check that the carb piston oil level is correct, it might be too low and allows the pistons to judder.
cheers
Robin

[nigelrbfurness]

Could it be carbs icing at constant airflow? I've expereinced that with other cars causing similar symptoms including engine stopping altogether!

Nigel Furness
Elan S4SE x 2, Jensen GT, A-H Sprite

[collins_dan]

Spent some time cruising on the highway at various rpm levels, and the issue is less holding a consistent rpm at high speeds, and more about hesitating on light accelerating at high speeds. So if I accelerate up to 4000rpm on the highway, then come off the accelerating, when I start to accelerate again, it hesitates (it feels like too much fuel) for just a couple of seconds, then accelerates fine up to any speed. It seems only to happen at high speeds in 4th gear on the highway, around town in lower gears, it decelerates and accelerates with no hesitation. Appreciate your ideas of things to check. Thanks. Dan

[collins_dan]

No suggestions? I've checked the archives and can't find any similar situations where the car runs really well in all situations except at high speed highway cruising, where it bogs and hesitates on acceleration. My lead idea is that it is drawing air through a carb leak somewhere, but where? Doesn't seem spark related as it's in such an isolated situation, doesn't seem fuel related as the plugs look great. Appreciate any suggestions of things to check. Thanks. Dan

[gjz30075]

Check that both air pistons move up & down smoothly - if they stick at any point, this can cause a problem...

Dan, is this an affirmative? What type of oil are you using for them?

Greg Z

[collins_dan]

I have fixed needle pistons and they seem to move fine. I use automatic transmission fluid, and check and refill every weekend. I'm not sure how it gets out, but it seems to always be present on the bottom of the carbs.

Dan

[ppnelan]

...it seems to always be present on the bottom of the carbs.

I would suggest this is more likely to be the residue from evaporated fuel i.e. a fuel 'leak'/weep, since the oil is unlikely to find its way down there...
Might be worth looking at in case it becomes a more serious leak - is your coil mounted in the 'standard' position below the carbs?

I use automatic transmission fluid, and check and refill every weekend. I'm not sure how it gets out...

Perhaps this is the problem - if the oil drains away every week, something is wrong...
Although you say the needles are fixed, it may still have pistons with adjustable needle height facility, including an 'O' ring 'deep inside' that lets oil leak INTO the carb air flow if it fails...

Matthew

[collins_dan]

I don't think its fuel on the bottom, I had the same concern as my coil is in the standard position. It's oil either being thrown up from the engine or oil dripping out of the carbs. I'll pull out my stromberg manual tonight and try to figure out the path of the oil. I don't think this is the cause of my problem as this has always been the case with these carbs, and the hesitation/bogging is a relatively new phenomenon, but I'm willing to entertain any theories at this point. Thanks. Dan

[collins_dan]

I've been thinking that the car is maybe running too lean, so got it running at 4,500 on the highway and it started breaking up, so I pulled the choke to see if that would make a difference (make it a little richer), but nothing happened. I did notice that when things start breaking up the tach starts bouncing around a bit between 4500 to 5500. Could this be a sign that it is electronic? Possibly distributor? It's strange. It doesn't seem to be about running up at high rpms, as I can accelerate aggressively up to 6,000 through each gear just fine, it just doesn't like highway cruising. Thanks for the ideas. Dan