Need help for Brat tunning...

[frankwelder]

Hi everybody,

I`m new on your site to try to get some advice. I`ve search on the net but maybe someone here may help me more...

 

Here is the short story;

I baught a 1986 Brat, Restored the body, and I put a new EA81 engine that has been modified for aviation. ( Camshaft and ported intake manifold, "conventional mods" ) I feed it with a weber 38 mm carb. I place my old distributor on it to get the vaccum advance (on aviation they remove it). I kept my original exhaust system in place.

 

Here is the problem;

Whatever where I set my timing, I have a hard time to ride from idle to 3000 rpm...engine bog and want to stop, but runs great from 3000 to 5500 rpm. I feel like the engine was modified to run at 5000 rpm and not at 2000 rpm...but someone may have some advice to me? Thanks

[asis]

Auto conversions as well as Rotax engines, most if not always run a gear reduction for the prop. Prop RPM full power cruise is usually only around 2600-2800 RPM mainly due to structural limitations and to keep the prop tip speed from ever exceeding the sound barrier. (approx. 720 mph @ sea level)

 

I don't know that this is the problem but it is a distinct correlation.

[GeneralDisorder]

I would guess that your carb is jetted wrong. I have no guideance to give you on which way it needs to go or what sizes to use as that is not a typical carb and you obviously have an atypical engine as well. The best thing I can tell you is that you will feel a lean condition more than a rich one and black smoke means rich. From the sounds of it I would say it's lean on the primaries but that is only a guess.

 

Best way to tune it - especially if you aren't experienced with carb tuning - is with a wide-band O2 sensor. Best $300 tuning tool I've ever bought (ok - so it's the *only* $300 tuning tool I've ever bought..... but it's still way neato ).

 

GD

[Mugs]

A 38 Weber is too big and you are "bogging" your engine down until it can catch up with itself. A weber 32/36 is what you want/need. Import Experts (click on link) have about the best deal around.

 

But then again this is for a "regular" EA81 and not the hypo airplane one you have. But I would say start with the carb and then go from there.

[frankwelder]

That`s what I was feeling too last time....I`m thinking of modifying my accelaration pump; Once I touch the throtle the pump feed some fuel. Maybe the pump should start is job at 1/8 of opening instead of the start. This may reduce fuel delivery from idle to 2000 rpm, reducing bog?

 

I`m not well jeted from idle mixture to 1/4 opening, rest is ok. The transision mode on weber carb is working from idle mixture jet? I don`t understand well this part of the carb...

[asis]

I wondered about the carb myself. I think that is the same one I had on my landcruiser. Is it the one that is running on both barrels all the time? IE: not typical primary/secondary? They call it "positive action" instead of "progressive"

 

That carb worked Sweet on my Cruiser but it was 290ci inline 6.

Edited April 17, 2010 by asis

[frankwelder]

Yes both barels open up together...they call it "synchronus" or someting like that...works very well from 3000 rpm to top end on my engine, but I need to try something to improve the idle to 3000 rpm bogging....Aviation camshaft was designed to allow more volume to goes in...works well on high revolution...I need to understand what happens on low speed operation...

[asis]

That is a bit of what has me wondering. I happen to be an A&P mech and also have 50hrs as a student pilot.

In aircraft, I am not sure this engine would ever run much under 3k RPM since (I know the Rotax for example) runs around 5500 RPM for cruise, which still needs to go through a gear reduction for the prop to turn >2800 RPM. With the same gear reduction, for the prop to turn 600 - 1500 RPM, needed for taxiing and run-up, and most importantly, LANDING the engine would still be turning around 3k.

 

Certified aircraft engine run the prop directly off the crank, so those engines do not (least should not) exceed 3k.

 

Again, I don't know for sure that this does or doesn't have anything to do with it...but it does have me pondering the possibility.

I can check some aviation forums and couple my pilots buds for some info along these lines... if no more than to satisfy my own curiosity

[asis]

100hp rotax in a Zenith CH-701 runs a 2:1 gear reduction. Prop RPM redline is 2700 engine is 5400. Idle speed for engine is 1800 or so.

 

I think your still looking at proper carburetion or at least jetting...

[naru]

I think your camshaft is unsuitible for low rpm operation.

[GeneralDisorder]

I think your camshaft is unsuitible for low rpm operation.

 

Wouldn't that affect the idle as well? Seems like if it can hold a decent idle then it should be able to handle progression through to high-RPM if the carb were jetted properly.

 

I agree it's probably a high-RPM HP type of cam grind with lots of overlap, but if it idle's ok then I'm thinking it's got to be jetting problems.

 

Wish I could help more - I haven't got any experience with the syncronous 38/38 Weber.

 

GD

[frankwelder]

Wouldn't that affect the idle as well? Seems like if it can hold a decent idle then it should be able to handle progression through to high-RPM if the carb were jetted properly.

 

 

GD

 

I`m agree with you GD, but since more volume goes in....it has an effect on progression....?...you think am jetted too lean? I feel more like too rich since too much fuel has to be burned and bogging result...no? I like your advice of getting the mixture sensor of 300$, but it doesn`t help me much more to understand what happen in my engine and cause the bogging....what would be your advice on timing? I can try different timing and tell what changes....Frank

[naru]

The aviation style cam gives less airflow than normal at LOW rpm,not more.

 

Splitting the diminished airflow in half w/a synchronous 2bbl results in an even weaker venturi vacuum signal.

 

I would try removing the gear that drives the "secondary" from the throttle shaft and running the carb as a 1bbl.

Since the Weber 38-27mm venturi is only 1mm larger than the 26mm Weber 32/36 primary venturi,this would result in low load venturi vacuum very similar to the 32/36 sucessfully run by many.

 

Or,try larger idle jets.

 

Some folks have reported running the Weber 38/38 on stock engines w/o major problems.

 

For curiosity`s sake-What is the manifold vacuum?

[asis]

The aviation style cam gives less airflow than normal at LOW rpm,not more.

 

Splitting the diminished airflow in half w/a synchronous 2bbl results in an even weaker venturi vacuum signal.

 

I would try removing the gear that drives the "secondary" from the throttle shaft and running the carb as a 1bbl.

Since the Weber 38-27mm venturi is only 1mm larger than the 26mm Weber 32/36 primary venturi,this would result in low load venturi vacuum very similar to the 32/36 sucessfully run by many.

 

Or,try larger idle jets.

 

Some folks have reported running the Weber 38/38 on stock engines w/o major problems.

 

For curiosity`s sake-What is the manifold vacuum?

 

From my memory (which admittedly sux) The weber 38 does not effectively have a "primary/secondary" at least not the one I have on my cruiser. Both barrels are in operation at all times off idle.....

Maybe more than one variation of the 38 weber?

[naru]

From my memory (which admittedly sux) The weber 38 does not effectively have a "primary/secondary" at least not the one I have on my cruiser. Both barrels are in operation at all times off idle.....

Maybe more than one variation of the 38 weber?

 

No actual "primary/secondary" (hence the quation marks),but one throttle shaft drives the other.

[GeneralDisorder]

I would try removing the gear that drives the "secondary" from the throttle shaft and running the carb as a 1bbl.

Since the Weber 38-27mm venturi is only 1mm larger than the 26mm Weber 32/36 primary venturi,this would result in low load venturi vacuum very similar to the 32/36 sucessfully run by many.

 

Or,try larger idle jets.

 

I agree with that. I think with the syncronous carb you just don't have enough vacuum signal to pull fuel from the primaries. Larger idle jet and maybe a larger pump jet might help.

 

Some vacuum/RPM numbers would be helpful.

 

I think you would have more luck with a 32/36. They can be jetted for upwards of a 3+ liter engine and are considered "upgrades" for the stock carbs on many. You wouldn't be splitting the vacuum signal at low RPM and thus it should run easier on the sequential style carb.

 

You might be able to get a kit for the 38/38 that would allow it to be converted to sequential. Ultimately though - that carb is really just too large for the EA81 - even with mods.

 

GD

[asis]

No actual "primary/secondary" (hence the quation marks),but one throttle shaft drives the other.

 

Ok I think I am following ya, but is each butterfly on its own shaft?

Otherwords, leave one butterfly shut and only allow one to operate?

[frankwelder]

Thanks for brainstorming my problem everybody....I`ll take vaccum/rpm reads tommorow and get back to you with it.

I was thinking about barrels today...Thinking of a way to make the second barrel to open up a bit later...I may try to work it on one barrel....but my stock exhaust is not a problem of working the carb on both barrels? I know it`s a big carb for this engine; I baugh this size in case of the engine would be use on something else than a Brat, such a "aeroslider" or whatever need lots of gaz and full throtle operation...Frank

[frankwelder]

Here is the vaccum reads taken from powerbrake vaccum fitting on left side of intake manifold...

900 rpm: 11.5

1100 rpm: 13

1500 rpm: 16

2000 rpm: 19

over 2000 rpm: still 19

 

When accelaration from idle, drops to 0 when the bogging occur and raise to 23 when engine accelarate ok....

 

If idle set to low (above 1100 rpm), when cooling fans runs and require alternor power...vaccum and rpm will go down together and engine will stall, but ok if idle is over 1100 rpm...

[frankwelder]

Since both barels work together, I adjust te idle on the first one, than I place the linkage back on (able to because of the slotting on the driver linkage)...so idle works only on one barels instead of both. It actually cut the problem by half...How should I jet barels since one is close and the other is working?

 

Can we post pictures on post?

[GeneralDisorder]

That is a SUPER low!

 

Normal engines run about 15 to 20 inHg (you are talking in inches of mercury yes?).

 

Does the needle move at all or is this a steady-needle reading of 11 at idle?

 

That would indicate either a manifold leak of some kind, late valve timing, or a really, really large durration/overlap on the cam.

 

If that reading is just because of the cam - I doubt you will ever make it run correctly with a Weber. You will probably have to go to something that has a double-venturi primary and is designed to operate off such a low vacuum signal. Most of the carbs I know of that are designed for that type of operation are 4-barrel V8 carbs - such as the Quadrajet.

 

The other option is to go to the EA82 SPFI manifold and run a custom fuel injection setup using something like Mega-Squirt. You could tune out that nasty bogging with injection.

 

GD

[frankwelder]

It inch of water, not mercury, and yes the idle is moving when I read 11 at 900 rpm, but steady over 1500 rpm.

 

I must run it like that since I`m out of cash for other big change...I can run it at 1600 rpm idle...But can I try somethings else with jets?

 

I should have kept my original engine and put a 32/36 carb on it....but I install that engine and I want to run it for couple years...

[GeneralDisorder]

I seriously doubt you are measureing in inches of water column - that would be a very low, almost non-existent vacuum signal at 11 inches of water. Inches of mercury is the standard measurement for vacuum gauges of this application and should show around 15 to 20 for a normal engine (29.92 inHg being complete vacuum). Conversion from inches of water is a multiplier of 0.07355 which would yield an inches of mercury reading on your engine of 0.8 inHg - far to low to even run.

 

The slightly moving needle at 11 indicates that it's probably just the cam.

 

My vote is for fuel injection. It's actually pretty inexpensive if you get a used SPFI system from an EA82 vehicle and adapt it. It might even run on the stock computer. At least it has the advantage of being able to compensate for engine demands via it's various sensors.

 

Jetting will not help a low vacuum signal. It's so low that the fundamental design of the carb makes it incapable of pulling fuel from the primary nozzles - the neccesary signal vacuum simply doesn't exist. You need a carb with a double-venturi and a vacuum/demand operated secondary to properly function with an ultra-low idle vacuum like that. You can try richening up the idle to *cover up* the flat spot somewhat - it will run rich at idle though. You also might be able to change the progression slots or holes to try and increase the amount of fuel flowing from the idle circuit durring throttle opening - but that will be a trial and error deal and any changes you make there are not going to be reversible - it will be easy to damage or destroy the carb body making changes like that. Increasing the pump jet size might help also though.

 

Whatever you do - it's going to guzzle gas like no tomorrow. Huge idle jets, huge pump jets, large progression ports, BIG primaries to keep it pulling up high..... it's going to suck to drive and it's going to suck on your wallet like a hoover.

 

GD

[frankwelder]

It already cost me a lot of fuel....boosting up idle, progression or pump will take even more fuel...but I know anything about the EA82 engine....can you tell me more about it? It was use on which years? You talk about sensors and computer...My Brat is running like a free bird without sensors, except the one on the dash board...But since I`ve adjusted idle on one barels, the car could be run like that, so I will take my pills for this summer, and try your mods next winter, but I will like to have your info about the EA82...

 

I will check my vaccum meter...but I`m pretty sure of the reads and the scale, but it`s an old sensor that I was using for fuel pressure....but If the vaccum reads are those ones; is there because of the engine cam mods or I may have a sever vaccum leak on the header...I will check it out with acetylyn torch instead of propane torch...

[GeneralDisorder]

It is typicaly of the older vacuum/fuel pressure shop gauges to read in Inches of Mercury (inHg) on the vacuum side of the scale, and then Pounds per Sqaure Inch (PSI) on the fuel pressure side of the scale. I have one of those from the 1940's hanging on my wall in the garage.

 

I wrote an entire article about using the EA82 SPFI on the EA81 engines. Here's the link:

 

http://home.comcast.net/~trilinear/EA81_SPFI.html

 

GD