Engine rpm is not stable below 2000 rpm

[MichaelBC]

Hallo,

yesterday I took my boat with the new engine out on the mediteranian see for the first time.
The engine starts immediately when I turn the key and runs nicely. I did set the timing and idle speed in setup mode of the ECU.

When I accelarate a little and reach about 1300 rpm the tachometer starts to go up and down by about 200. I can also feel and hear it. This behavour continues until I reach 2000 rpm. From there the needle of the tachometer is stable like glued there.
Has anyone of you an idea what could cause this between 1300 and 2000 rpm ?

This is the setup
Mercruiser 4.3, factory new
Edelbrock carb, new, configured for 4.3 out of the box
Thunderbolt V ECU, bought used
New spark plugs, fuel filter, distributor cap and finger.
Timing set to 10° advance in setup mode

Regards

Michael

 

[Lou C]

Sounds like a Vacuum leak, are you using an adapter plate under the carb? bolts tight? The 4bbl manifold (cast iron) is a spread bore pattern (for the Quadrajet carb used before the Weber/Edelbrock style) and the Edelbrock is a square bore carb, usually you need and adapter plate in that instance.

 

[MichaelBC]

Sorry, I forgot to mention that there is also a new Edelbrock intake manifold on the new engine. That should match the carb shape.
Thank you for your hint.
I think I did plug all unused openings in the manifold. But I will try to get some break cleaner tomorrow and spray the outside of the engine. If there is a leak then idle rpm should rise.
Checking the bolts is also an easy task that I will do.

 

[alldodge]

Edelbrock carb, new, configured for 4.3 out of the box

No carb is pre-adjusted out of the box. They all need adjusting, some more then others

Motor warmed up
Timing adjusted
Adjust RPM and keep it there as further tuning is done
Adjust idle screws for max vacuum

The dead spot as RPMs increase is the transition from idle to Primary jets. May need to increase accelerator pump rod stroke

 

[MichaelBC]

With configured out of the box I meant that the needles and jets have been changed for the 4.3 already.



Of course I did set the idle speed to 650 rpm and the timing to 10° advance. Both with the ECU set to adjustment mode.

So all adjustment that I can do on the carb are the idle mixture screws. Initially I turned them in until rpm dropped and then turned a little back again. This equals in about turn them in all the way and back for 1 and 1/2 turn. I tried also a richer setup and turned back 2 or even 3 turns. This doesn't change anything on the unstable rpm in the area between 1300 and 2000. The accelerator pump lever is in the position where it gets the most fuel. But this has not effect as I do not accelerate but run in constant speed when the rpms go up and down.

 

[Scott Danforth]

The 1409 normally needs the 1481 calibration kit to work with the 4.3

 

[Scott06]

Probably lean on the idle transition circuit. Back out the idle mix screw a half turn see if it improves. I think several folks here did not like the edlebrock 4.3 calibrations @Rick Stephens

rick may be able to point you to correct calibration, but generally the main venturis take over about 2000 rpm, at lower revs you are on idle and idle transitionion circuit

 

[Rick Stephens]

Your results may vary, several of us on the forum found the 1409 with the 1485 V6 metering kit installed to be severely lean in low through mid range. I had to jump the jets, rods and springs up to find safe enough margins not to burn holes in pistons.

 

[MichaelBC]

Hallo,

thank you all for your answers. If I google for Edelbrock 1481 then I just find a throttle lever kit which seems to be somehow wrong.

1485 shows springs and rods for the 4.3 liter engine. So if this is to lean, could you recommend me the kit number to order ?

Regards

Michael

 

[nola mike]

Your results may vary, several of us on the forum found the 1409 with the 1485 V6 metering kit installed to be severely lean in low through mid range. I had to jump the jets, rods and springs up to find safe enough margins not to burn holes in pistons.

And I was severely lean running Rick's setup, even though we have the same engine. The recommended calibration kit (forget the numbers off hand) I think is for the pre-vortecs, and possibly the vortecs need a richer jetting. FWIW, Michigan Motorz uses the 1409 out of the box (which is allegedly jetted for a 5.0) for their new 4.3's.

 

[MichaelBC]

Would the 1488 kit include the jets and needles that I need to make it more rich ?

 

[Scott06]

https://static.speedwaymotors.com/pdf/325-1403.pdf

this is the tuning guide. Look up the page for 1409 although not sure what jets yours came with fir the 4.3 calibration the graph shows how to make different phases of operation richer or leaner. Process of making a change reading the plugs then repeat…

 

[MichaelBC]

Thank you very much. I think this will help me.

 

[Rick Stephens]

There is no kit that directly takes you to the correct calibration. The 1485 is way too lean for anyone I ever talked with. I spent a bit of time trying individual rods, jets and springs until I found what worked for me. A number of others have used this final setup successfully. And nola mike tried and was still too lean - the only person I know who did not find this setup to work.

Here's the post oof where I ended up:

Anyone duplicating this on a Edelbrock 1409 mounted to a 4.3L Vortec V6, don't bother with the 1485 kit. That thing is not even close. Might be good on a pre-Vortec, but is way too lean on a Vortec with a Performa intake. Here's where I ended up

.095 (part#1426) primary jets
.092 (part#1425) secondary jets
.065" x .057" (part number 1461) meter rods
7" pink step up springs

 

[MichaelBC]

Hello Rick,

I changed my carb to your setup last night. As I am on holiday in Croatia and there are not parts available here I did order in Germany and had them shipped to my friend and he brought them for me yesterday.

I had to take what I could get quickly and so I bought a kit and the rods. The kit contains the jets that you mentioned.

Today I did a test ride and it felt like a completely different boat. The engine has a stable rpm below 2000 and when I push the throttle quickly there are no hickups anymore. I had this before that opening the throttle quick the rpm first when down before it did increase. Once the engine even stalled. This is all gone now it pulls right up from idle and it also feels more powerful.

So I owe you a very big THANK YOU !!!

In the original V6 kit the primary and secondary jets are the same .89.

The rods were marked with 6857


Springs were pink


So I bought this tuning kit for the 1409


It does not contain the rods you mentioned. Luckily a found a shop who had them on stock.


So I chekced the float levels that were ok and the carb was clean inside.




Thank you again for solving my issue.

Best regards
Michael

 

[Rick Stephens]

Great news! And you are most welcome.

Rick

 

[nola mike]

Very strange. I'm still having similar problems to you at transition rpms (but I'm rarely in that range so not enough to do anything about it). I ended up basically back at stock settings and it runs well now. I have a bit more tweaking, but I'm pretty damn close. Did you end up using Rick's setup exactly?

 

[MichaelBC]

Yes, I used exactly what Rick suggested and I am very happy with it.
Both jet sizes were in the calibration kit 1488 and so I only had to boy the rods seperately.

 

[Lou C]

thanks for posting this up, I am a loyal Quadrajet fan but if something happens to this one there really aren't a lot of good cores left so being able to use the easily found 1409 is a nice option. I tried the Holley that OMC and Volvo used in place of the Q-Jet and it never ran right so I took the Q-Jet apart and carefully cleaned and rebuilt it. So far so good. I still have that Holley, I took it apart a few months after I re-installed the Q-Jet and I THINK I fixed what was wrong, but am leaving the Q-Jet be for now....