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Engine Dies When Blipping Throttle


Best Answer jeffster06 , 10 February 2019 - 01:37 AM

Tested lifting the piston with my finger, it didnt offer a lot of resistance. I swapped the oil for some 20w50 (heaviest in my garage) the piston was heavier, but I wouldn't say it was hard to move. Took it out for another drive and it didn't stall when I blipped the throttle, it stumbles pretty hard but recovers (goes to about 900 rpm then back up to 1200 where my idle is set).

I might try some 75w90 gear oil next, but it seems I'm on the right track. Go to the full post


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#1 jeffster06

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Posted 09 February 2019 - 03:15 PM

I recently rebuilt my 1275 (has around 1000 miles on it now) and was having lean issues changed the needle from a BBW to a BDK, checked fueling with a colortune and have a nice blue flame when idling and holding at various RPM's. The issue I am having is since I changed to the BDK if I blip the throttle it wants to die, sometimes I can save it by continuing to blip the throttle and if I cruise for a bit it will be back to normal, if I blip the throttle to downshift coming to a stop it will try to die. I did not have these issues with the BBW. 

 

Some build info

1275 overboard to +40 (so 1310?)

minispares evolution001 cam

minispares evolution dizzy, the one they recommend for the cam

MED stub stack with foam filter

Hif44 with yellow spring (putting the red back in makes the issue worse it seems)

BDK needle.

Minisport 1.3 roller rockers

Minispares vernier timing set (set close to 107 per their recommendation, best I could get was 106.5 ish, I did the math multiple times then averaged what I was seeing). 

stock head with larger inlet valves, and minispares double springs

 

Prior to swapping the needle I had bottomed out the enrichment and was still having issues running lean, when cruising on the highway in 60 ish weather my coolant temp would rise to 195 and I would need to use the heater to keep it down in the 180 range (I have a 167 degree thermostat). Other than the temp issues on the freeway overall driveability was good. plugs were white in color, along with the temp I figured I was leaning out around 3000 - 4000 rpm. 

 

With the BBW the car runs amazing aside from the wanting to die after blipping the throttle. I rechecked my timing and had my wife hold it at 4000 RPM and verified im at 32 degrees. I have verified the dash pot is full, I use the SU dashpot oil in it. I sprayed around the carb with wd40 as well to see if I had a vacuum leak, the revs did not increase. 

 

Any thoughts on what I should check next? 



#2 Moke Spider

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Posted 09 February 2019 - 04:27 PM

An obvious question here to ask is have you checked the Dashpot Damper for Oil and what Oil do you have in it?

 

If that's OK, often with HIF44's the Damper Piston needs a swap out, there are a few different types giving different damping on the rise vs fall characteristics.

 

Without a rolling road to set it up on, with oil in the Damper, if you fell the resistance needed to push the Dashpot Piston up, usually they need a fair bit of resistance needing quite a bit of effort. If you find it rises very easily, then try a different one. Without access to the myrid range of all the Damper Pistons available, you can swap it for one from a HS4 or HS2 from any of the early Mini Range. While it may not be optimal, it will likely give the added damping (and acceleration enrichment) that is needed or may in fact be a little on the rich side, but that will only occur while acceleration moderately to hard. Once you get back on the Cruise again, this will have on impact.

 

The Damper Piston is held in place with a tiny E clip and take note of which way the Piston needs to be fitted.



#3 jeffster06

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Posted 09 February 2019 - 04:57 PM

Checked and topped off the damper oil, I'm using SU oil not sure on the weight I can check though. When you say move the piston should I remove the dashpot or just take the filter off and try lifting it with my finger. I want to say lifting it was not very hard, but I will verify this. I wish i had someone around to tune this correctly on a rolling road, but where I'm at theres not anyone around, at least I havent found someone.

#4 Moke Spider

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Posted 09 February 2019 - 05:59 PM

I would suggest engine oil (it's what BMC / Leyland / Rover always recommended) and for that matter, even try gearbox oil. If I recall, the stuff SU sell is only an SAE 30.

 

It does some like it's leaning out on acceleration and this will help, though, it may or may not cure it.

 

Yes, the damping on the Carb Piston with the air filter removed, finger in and,,,, liiiiift,,,,



#5 jeffster06

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Posted 10 February 2019 - 01:37 AM   Best Answer

Tested lifting the piston with my finger, it didnt offer a lot of resistance. I swapped the oil for some 20w50 (heaviest in my garage) the piston was heavier, but I wouldn't say it was hard to move. Took it out for another drive and it didn't stall when I blipped the throttle, it stumbles pretty hard but recovers (goes to about 900 rpm then back up to 1200 where my idle is set).

I might try some 75w90 gear oil next, but it seems I'm on the right track.

#6 hhhh

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Posted 10 February 2019 - 02:35 AM

The gear oil is about the same viscosity as 40 to 45 weight engine oil. Since the the 20w50 probably never gets hot enough to get much above 20 weight, the gear oil should be about double. If you mix the two evenly, you should get the magic SU oil number.






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